The Imperial City is divided between the Imperial Sewers and the six Imperial Districts. Each Alliance has a small base in the sewers which provides quick access to the districts, but getting back to base from the districts requires traversing through the sewers. Tel Var (the currency of IC) are earned and used within the City, but can be lost by dying (you lose half when killed whether it being a PvP or PvE encounter).

You can enter the Imperial City through the Campaigns Tab [Default Keybind: L] and selecting either the CP or Non-CP campaign. Once selected you'll be queued to join the zone.
To leave the Imperial City, you can do so two ways. You can either queue for Cyrodiil and then leave via Wayshrine OR you can leave via one of the doors in your Alliance Home Base within the Imperial Sewers. The latter option will take you back to the last PvE zone you were in.

Among the Imperial City Districts (sometimes called Top, Top-Side, etc) are 6 flags, one for each district. Capturing these flags flips it to your faction. Upon capture, you gain a small amount of EXP & AP whilst also summoning guards from your faction to help defend it. While the district is under a faction's control, any player within the district that is with that faction (DC, AD, or EP) recieves a 33% bonus to Tel Var earned from killing mobs. However, this bonus does not apply when killing enemy players. The bonus is additive depending on the amount of districts captured. (198% bonus TV gained if all 6 are captured). Patrolling around each district is a "Horror" or boss. These bosses drop a significant amount of Tel Var and are often sought after by farmers. These bosses have arguable ranges of difficulty as some have various abilities that deal high damage, CC and/or regen HP. Farming these solo is an option so long as your build is cut out for it, however farming in small groups of 2-3 are more common & efficient.

Within the Imperial City Sewers is your alliance's home base. The base has a myriad of shops, merchants, and crafting benches for players to use. Outside the base is a maze of areas containing high mob density & patrolling Bosses. These bosses are usually ignored as the amount of Tel Var is quite small in comparison to the bosses top-side. The main event found in the Sewers is located in the center, the Barathrum Centrata, where players are able to fight a copy of Molag Bal. This version of him has exclusive drops such as the Soul Shriven skin and is often seen being farmed by groups. Despite this, the sewers is mainly seen being used as a region for leveling new characters as the density & respawn rate of mobs is high.

Tel Var is the currency found within Imperial City. It is used to buy items found only in the IC DLC. You gain these stones from either killing mobs, bosses or players. When carrying these stones you'll see a Tel Var Counter (blue in color) that shows the amount that you're carrying, as well as, the current multiplier that your character has. The multiplier, including any bonus % you get from districts captured, determines the amount of tel var dropped from mobs.

The multiplier values are:
1x 1-100
2x 101-1,000
3x 1,001-10,000
4x 10,000+

Useful Items & Sets you can buy using Tel Var:

Black Rose
Galerion's Revenge
Imperial Physique
Meritorious Service
Phoenix
Powerful Assault
Reactive Armor
Shield Breaker
Thews of the Harbinger
Vicecanon of Venom

Hakeijo (used for Prismatic Glyph)
Waxed Apothecary Parcel (used for acquiring Alchemy Mats)
Storage Chests (housing item with 30 & 60 slot variants)