A quest is a task or duty which a player must accomplish in a role-playing game, usually in order to advance the plot of the game. The task in a quest is defined by specific objectives which the player must complete. Upon completing the quest, the player may receive a reward or simply progress to the next part of the story.
A side quest is an optional quest which the player does not need to undergo in order to beat the game. Side quests generally serve to gain extra money or experience, or more powerful items or equipment.
Too often, quests are fetch quests, sometimes a tedious pause in an otherwise great game.