OK getting a bit tired of having to individual teach every single player I dungeon/raid with how/why to do the most effective healing, so here's a complete guide:

Healing 101, applies to pretty much every mmo:

The #1 best way to keep your party alive is to avoid the damage in the first place.

Most important part of that is strategy - stand in the right place and live, stand in the wrong place and die. 2ndary, and key only in a few places in this game is roll dodging at the right time.

Another key to this ideology is buffs - while ESO is very lacking in this department, as a healer you do have access to one extremely strong buff - Combat Prayer. Use this as often as humanly possible. It's a morph of Blessing of Protection - restoration staff line. It greatly enhances your parties armor, spell resistance and most importantly - damage. Why is damage import to healing/damage prevention? Dead monsters don't hurt your party, damage makes them dead.

Racialist choice:
Races in ESO are not equal, and the differences can be noticeable:

The best healer race by far atm is Breton. Reasons:
- Reduced magicka costs by 3%: This bonus can not be replicated by any other, and does not get diminished by the overcharge system.
- +10% max magicka: This not only increases your pool by a noticeable amount, but also increases the strength of your heals as in this game: Magicka adds to spell power.
- Increase spell resistance and faster light armor leveling: Doesn't matter much, but they aren't bad things to have on a healer at all.
- Breton girls are the most attractive, and best in bed.

Other viable choices:
Altmer:
-High elves are extremely evil and unattractive, but they do also get the same +10% magicka as Breton. Though their other racials don't really apply to healers. As: Increased recovery is essentially completely irrelevant in this game at endgame (too overcharged, too little to have impact). Though if your planning a sorcerer or DK healer, their bonuses to fire/cold/shock damage will be useful.

Nord:
-As of the latest inexplicable update, Nords are now the toughest race (LOL GOTCHA if you paid 20 bucks for the previous toughest race).
Why good:
-Not dying means you can heal for longer. You won't be as attractive or as sustainable a healer as a breton, but you'll die less.

Argonian:
If you want to be a gross lizard thing, you at least get some minor healer benefit for doing it:
+15% potion effectiveness means more mana back on mana pots.
-Quick to mend means you may die less, though if your that strong of a healer anyways, generally your over healing yourself regardless.
-Other benefit is Argonians are Ebon Pact, which everyone knows is the best alliance with the most attractive guilds.

Class:

On first glance one may think only Templars need apply, but this is not the case. As to be the most effective at burst healing, yes templars win.. But ESO healing, when playing with GOOD players is not so difficult to require that, even in the hardest trials.

So other classes first:

Dragonknight:
On first glance one may think them to be weak.. But actually in many situations, they can be the best healers in the game. Reasons:
- Molten weapon increases resto staff heals. More so then any other classes bonuses.
- Banner increases heals too. +20%
- Ingenous shield is the largest buff to resto staff heals in the game. period. +30% is massive. Mana cost is high enough that it's not something you can spam crazily, but with smart use of Spell Symmetry you can maintain it in many scenarios.
- Their pretty freaking tough, higher block mitigation then all but Templar. Toughness is important on a healer, as heals generate agro - and a lot faster then DPS does in this game.

Con:
- More difficult mana management then Sor/NB. But with frequent Battle Roars, not that bad at all.


Nightblade:
Possibly the easiest to play, as they can get back mana really easily.. But also imo, the least effective. ESO requires FAST and STRONG heals to survive. Nightblades can really only provide the baseline amount. Realistically atm all good NB endgame players actually heal to some degree, but thats due to weird game design requiring them to heal some to actually do dps. As a primary healer role, I'd recommend a skip unless your a very casual player looking for an easy time and not going to try the hardest content.

Sorcerer:
One of the most common, and for good reason: They have some resto synergy, and strong mana management skills.
- Critical surge provides a decent increase to weapon damage, and thus restoration staff healing.
- Dark exchange gives you unlimited magicka without hurting yourself, as stamina is in most cases, not important to you as a healer.
- Class passives are bordering on OP: EG: -15% ultimate costs.

Templar:
Well, that's my main so I have a lot to say about this pretty messed up class:
- They are the only class with a full healing line as one of their 3 class trees. And this line does have the most powerful/best burst heal in the game: Breath of life. However the rest of the line is bordering on worthless atm due to frequent stealth nerfs. I'll go thru each one:
- Healing ritual and both morphs: 100% worthless. I refer to this one as the "clap" as you clap your hands together, and generally look like *** and do *** low amounts of healing in a stupidly long animation. Essentially this skill will get you killed a lot more often then it will save anyone, as you can barely move while casting it (unlike nearly every other skill in the game). It's an AOE heal sure - but so is healing springs, which is around 9000% more effective. (and breath of life is multi target too)
- Restoring Aura: Meh. Repetance is an ok morph for pvp if your massively slaughter your enemies. otherwise it's very skippable. It gives stamina, which is important, but Luminous shards (Spear line) is a vastly superior stamina heal.
- Cleansing ritual: Meh. There is exactly 1 fight in the game I can think of where having a synergy based cleanse is useful: Banished Cells end game. And actually your not meant to do that if you want the achievement there.. So um, yea. However due to it's massive aoe and the fact is provides a (sometimes functional) +30% boost to breath of life, it is worth slotting.
- Rune focus: Last I tested, if you cast this - it will REDUCE the healing you provide rather then increase. Severely bugged and not worth it.

Ultimate: Get practiced incantation.. As remembrance was not only mega stealth nerfed in the most recent major patch, but also broken: Plain doesn't work around 50% of the time.
Incantation is arguably despite the frequent nerfs still one of the most useful ultimate's in the game. It can get you thru the hardest raid bosses special attacks.

So yea, the Restoring Light line is useful. It renders templar healings as the #1 best burst healers in the game.

Though as I explained before, thats not really necessary. You'll notice the world records for the trials include zero templar healers for good reason: Templars do horrible dps and have horrible mana management skills (none at all).

Essentially if you want to play a templar healer and not suck, you MUST get mages guild leveled fast and unlock equilibrium/spell symmetry. Yes you do suck if you don't have it im sorry but its' true. Unlock that, use it often, don't use it at times that will cause you to kill yourself, and you'll be a very strong templar healer.


So yea overall shortform recommendations:

Low skill level required/easy mode/still win the game/best at trials:
Sorcerer
Moderate skill level/more burning/more dps at close range:
DK
High skill level/strongest burst heals:
Templar
Want to make the game harder then it needs to be:
Nightblade

Build:

Attributes:
49 into magicka. Enchant armor with +health.

Most beta guides recommended 49 into health, as in beta health was a 2 to 1 ratio, thus plain better. This is no longer the case. Do not try to build your healer as a tank because some 4+ month out of date guide said so.

Magicka is the correct choice for obvious reasons, but I'll explain at least 1 anyways:
If you do say easy content, where you dont need a lot of health.. You could carry a separate armor set enchanted with +magicka, and thus be 100% maxxed out for healing, and possibly able to keep alive the most *** players ever. (and more importantly, deal more dps while you do that). Vs if you did health: Well no, your party probably won't wait for you to hit the respec shrine when you do an easy dungeon.

Minimum health for trials: 2300 - 100% fully buffed. If your at the top 1% of player skill, and run in a top tier guild of like players, you can survive at 2200.
(0 points in health you can still hit 2300 without much effort, in fact you can hit 2400. Just enchant armor, use emperor alliance buff and food)

Skill Morphs:

Sor specific:
As I mention before: Get critical surge. Not much else is specific to healing so won't go thru a whole sorc guide here.

DK Specific:
As I mention before: Get igneous shield. Also get Igneous weapons. Not much else is specific to healing so won't go thru a whole DK guide here.

NB:
Get Funnel Health. Do NOT take the healing morph on Consuming Darkness. Veil of Blades is by far the most powerful ultimate in the game, get that, keep it up, it will save your party from death a lot more then the heal morph will (Dead monsters don't kill your party remember)

Templar:
I went into a speil early, so just a quick rundown here, read back up top if you want explains:
Restoring Light Tree top to bottom:
Practiced Incantation. (Mostly because Remembrance is currently broken)
Breath of Life
Skip this junk (h ritual)
Optional (aura morph for pve, repentance for pvp)
Extended ritual
Optional (Channeled focus if you decide yes)

Dawn's Wrath:
Solar Prison (Use this when heal ult is not necessary)
Skip entire rest of line, complete trash.

Aerdric Spear:
Biting Jabs (Primary dps for when required to not heal: EG Boss 3 of Archives should only have 1 healer, yet 2-3 are useful on other bosses, so here some healers need to switch to dps)
Skip (Javelins are the suck)
Optional (Focused charged is ok when your fighting complete fools in pvp)
Luminous Shard (EXTREMELY powerful, 25% +~220 stamina heal and +220 mana heal. Toss this to your tank in trials, it will help him immensely)
Skip (Sun shield = bad skill, even for tank templar)

Weapons:
Restoration Staff:

First off Resto staff 101 since the game never explains this:
- A heavy attack with a restoration staff will restore around 15-20% of your mana. This is insanely important to use often in cases where you cannot safely use equilibrium/spell symmetry.
- A higher weapon damage rating on a resto staff not only increases the damage the heavy attack deals, but also how much you heal for. So ask for a molten weapons buff, and make your staff legendary ASAP.

Skills:
Grand Healing --> Healing Springs:
The other morph is plain horrible compared to springs. Springs in a trial = nearly free due to the mana restore. Even in a 4 man, if you can manage to catch all 4 players, it's very cheap. And the extra tic the other version gets is worthless.
Why: Healing Springs stacks with ITSELF. So you can and MUST spam it over and over to make it effective, thus the 2/3/4th tic while they still function - are not critical to save your party members, the more you spam, the more people die less. It's used for burst AOE, and for more burst, less mana cost is neccesary.
Regeneration --> Rapid Regen:
This is really not very a useful skill at all for difficult content. So you could completely skip it. However in terms of mana efficiency, it cannot be matched. Why not the other bad morph? Because you have to be really bad at this game to hit 20% health, and in anything actually hard, you'll be 1 shot from 50% health anyways - thus the trigger never really helps. In 99% of trials/dungeons: Do not even slot it. It's actually most useful for zerging tho: If your doing trial speed runs, its good for alt bar use - to run by trash. Also for soloing.
Blessing of Protection --> Combat Prayer. See healing 101 at the top.
Steadfast Ward ---> Skip. It's ok at low lvl. Bad at endgame, does not scale correctly.
Force Siphon ---> Skip. Was nerfed too heavily into the ground.

Other useful things in other trees:
Light Armor:
Annulment: Die less to magic
Hvy Armor:
Immovable: Die less to physical attacks. (still wear light armor, works fine in it)
Mages Guild:
Get all passives. Get Spell Symmetry. Use it, love it. Win the game. I mean think about it: It gives you fast unlimited magicka. Thats is as useful as it sounds, and actually even more so.
Alliance War:
GET EVERYTHING. Trials are mega unbalanced, most skills only hit 6 players, Alliance war ones hit all 12. Thus are all useful.
Most important:
War Horn - Aggresive Horn. Big buff to healing and damage for everyone. Often more useful then your other ultimate's in a trial.
Barrier: For a while I thought it critical, but eh - top skilled players can complete trials fast and without deaths without it.. But it does allow for sloppier play. Recommended. Sloppy drunken play is more fun.
Retreating Maneuver: Running fast = Less dying. Very good.
Purge: Eh. They messed up the trials and forgot to allow you to actually purge much of anything. Though it is useful in pvp (works on siege weapon DoTs)

Gear:
The most common mistake players make here is jewlery.

STOP USING THE GARBAGE WARLOCK SET ALL CAPS.

- The correct choice for jewlery is +spell power. Spell power provides a MASSIVE (around 35%) increase to templar healers. And while that may be overkill on the primary hit of breath of life, on the 2 smaller hits , it is not, and it will save your party many many many times. Get it. Use it. Love it. (Also applies to your ultimate heal, which is critical in the trials)

Non templars:
- Correct choice is +weapon damage. This will greatly enhance your resto staff heals. Which on their on are strong, but having them be even stronger WILL help.

Mana management, is done via spell symmetry/Dark exchange/syphon/battle roar. Not gear. Don't ruin your gear because you can't be bothered to install the lore books addon.

Another common noob mistake:
"Should I stack crit?"
Crit is low priority for 1 clear reason:
You can't say to someone "oh well around 5-50% of the time I'll heal you enough so you don't die". Healing has to work 100% of the time. Thus you never rely on criticals.

That said, crits are necessary in this game not for the extra healing, but instead for the ultimate generation.

However simply running magelight (innerlight) and setting your campaign to one with the +8% is plenty (36% total with precise resto staff). Just pop a spell power/crit/magicka pot at the right moment when you need more (pretty much never, but can help in lower powered grps in trials).


So correct endgame gear setup (and I do mean correct, this games is far too unbalanced/difficult to offer "choices"):
4 magnus armor + magnus resto staff
3 aether armor (Trial set) or before you acquire it: 3 seducer
All light.
Jewl:
Buy from vendor in craglorn - v12 blue arcane rings. Neck there has health, which is fine, else get a random version from zone chat with arcane. Enchant all 3 with +13 spell power if your a Templar. +13 weapon damage if other class.

Build:
49 in magicka. Enchant the armor with +health. Aim for a minimum of 2,300hp in trials. If its lower due to lack of emperor buff or something, try a healthy trait necklace, or 5 in health, maybe 10 in health, maximum.

Armor traits:
3 impenetrable. (30% less damage from crits)
4 infused (extra health)

Why defensive traits:
The offensive traits add zero noticeable effect. Players will not live or die based on you having a few extra magicka or +0.1% healing from divines+ritual stone. You however WILL get 1 shot by frost breath in the aetherian archives if you do not have impenetrable traits.

Non crafted upgrades:

Aether as I mentioned. It drops in either raid. Not that hard to get 3 piece, as 7 pieces drop total, and a single completion for the weekly nets you an 100% chance at 3 raid loot pieces. (1 boss drop, 1 quest reward, 1 mail reward).

1 other possible upgrade is using the healing staff of thorn +2 ring set bonus (rings have arcane iirc, and are enchantable)..
Issue with that however:
staff is v10. Set bonus is only +8% healing. So you'd lose some ~2-3% Resto staff heals in the process.

Larger issue: Rings are a 1 in 1,000,000 drop rate and no one runs vr dungeons anymore. Impossible to acquire.