Proc sets are any sets that activate a damaging or healing effect based on chance or a specific action. Basically, they add free damage to the regular active skills and passives you use. PvP is currently plagued with builds that put together 3 proc sets, which can actually allow a simple light attacks to also activate three procs and instantly kill someone.

they have been around since the game went live - see ashen grip for an example and death wind, witchman/shadow, warlock etc - though some might disagree with death wind since its effect is a stun and knockback not damage.

With 1T more of them were made readily available overland and some available in dungeons and such along with monster helms are allowing for combo bursts that reach quite high level - even if the character build is mostly defense and survival.

Why do people hate them?

First law of Nerf-o-Dynamics.

For me the design flaw is that the character can be built entirely defensive - heqalth/hvy armor - and the set piece effects dont change (except for Cp impact.) IMO if the set's "effect" was half from set and half from character's wpn/spldam (basically letting set quality/lvl replace the magica/10 factor) they wouldn't be as troubling.

haha a problem brought about by PVP players who argued and complained constantly about having to grind Cadwells so they introduced 1T to make it easy for them.