For quests in each main release, visit their individual pages.
Quests are missions within the game. These help to advance the plot and storyline of the game. The main quests are compulsory missions and without completing them, the game and storyline cannot continue.
Side quests are optional missions which are not required to complete the main questline. They are typically started by speaking to non-player characters, although some can be started in other ways.
An errand is a type of side quest that typically has weaker narratives and requires less direct action on the part of the player.