Quests are additional source of income (Gold, EXP, Gems, Creatures etc.). To complete quest you have to complete certain task (that mostly isn't that difficult).
Quest for collecting Gold forms set of tasks: Collect 7500 Gold — Collect 37500 Gold — Collect 150 000 Gold.
There are 3 type of Quests:
Epic. Their achievement is time-consuming and as a reward you receive elite T3 or T5 creatures or Gems. Epic quests are finite and to complete all of them will take lots and lots of time.
Main. Main quests form long sets (i.e Achieve Rank or Clear Location), they are finite and as a reward you receive small amount of Gold, Resources, Gems etc.
Daily. Daily quests update 4 times per day: 09:00, 13:00, 17:00 and 21:00 (UTC/GMT +3). Reward for completing most of them is EXP. Some daily quests give you Gold or Gems as a reward.
Quests are formed chaotically (means that "Login daily certain amount of time" quest may go after "Use items" quest). Rewards are: Gems, Food, Soulstones, Gold, Rage, Wood.
There are 2 regular sets of tasks:
Achieve Rank (from 2 — 100)
Clear Location (counts first victory on Normal difficulty)