Quests
From Diablo Wiki
In Diablo I, the quests are a series of missions (fewer in multi-player) on which the character is sent, and which offer advantages when completed. None of them need to be completed, but all add background detail and most give valuable rewards (even if only when reselling).
In Diablo II, the quests are a series of missions of which most are integral to proceeding in the single-player game. For example, without the Cube, the Heart, Eye and Brain, and the Flail, the single-player character cannot get beyond the Third Act City. However, in multi-player, an advanced character can open a town portal from a waypoint beyond the blocked part, enabling a less-experienced player to bypass the quest entirely.
