Quests are simply tasks which Scarlett can perform in order to succeed in her main goal, a mechanism Venetica uses to help organize activity. Some quests are quite simple and straightforward; other tasks may require many steps. Some quests will spawn other, subordinate or sequel quests. Some Quests require other, prerequisite quests be completed first, others can be done in any order.
Scarlett will probably have many quests active at the same time. The games inventory screen provides an "Active Quests" page which lists each quest Scarlett has received but not yet completed. Selecting a specific quest reveals a journal page describing the details of the quest and what Scarlett must do next to continue that quest. Some quests will remain active for a very long time, as Scarlett will need to deal with many prerequisites first. The player can activate "show on map" for any Quest to show the next location Scarlett should visit to pursue that quest: a small green dot appears on the map (and inset mini-map). Some Quests may show multiple locations, especially if it doesn't matter which order Scarlett visits them.
Many quests are mandatory: Scarlett must complete them in order to succeed in her primary goal of defeating the Undead Archon. Other quests are optional: Completing them will add to Scarlett's Experience and Reputation.