Campaign Mode (sometimes referred to as Story Mode or simply Campaign) is one of the game modes in Diablo III, the others being Adventure and Apprentice Mode (consoles only). Prior to Patch 2.0, Campaign was the only mode available to players. It is focused primarily on narrative, whereas the Adventure Mode is a preferable choice for farming.
In Campaign mode, players progress through the game one quest at a time, with all scripted events fully functional. Once a boss fight or a plot-relevant event has been completed, it will not appear in subsequent games, unless players manually reset the quest and choose to continue from an earlier point. However, completing a quest for the second time will award less experience and gold, and bosses will drop less loot. Quests may not be skipped, although players are free to choose any quest to start with from the quests they already have completed.
Waypoints may only be used to travel within a current act, and only among those that already have been unlocked. All dialogues are opened, as according to the current plot section. Adventure Mode NPCs cannot be accessed. Artisans' carts may be used for crafting, though, even if plot-wise that character did not recruit the respective artisan yet: it is enough to unlock them at any other same-type character on account. Some Super Unique monsters will only appear in Campaign Mode.
Completing the campaign (that is, killing Diablo for the base game or Malthael in the expansion) awards one guaranteed legendary item. In the expansion, killing Malthael for the first time will also unlock Adventure Mode for that player's account, if it is not unlocked yet. The guaranteed legendary only drops once, on the first kill. To get it again, one must manually reset the whole campaign (using the foolproof "Reset Quests" button), starting the campaign all over again from the Fallen Star quest. Resetting the campaign does not block the Adventure Mode, but it is an irreversible decision. Note that the four companion quests (personal quests of Eirena, Kormac, Lyndon and Shen) may not be reset; each character can only complete them once, even if they start a new campaign.