Missions are tasks given by NPCs or items located throughout The Secret World. Press J to view your mission journal and click a mission icon to display detailed information on a specific mission.  Current missions also appear in the mission tracker on the right side of the screen.

You can have 1 Story Mission, 1 Dungeon Mission, 1 Main Mission (Action, Sabotage, or Investigation), and up to 3 Side Missions at a time.  Story Missions tell the larger story of conflict growing in The Secret World and how you and your secret society factor into that conflict.  Dungeon Missions challenge you to defeat difficult enemies with a group of other players in dungeons.  In Main Missions, NPCs may send you to defeat enemies, ask you to use stealth to infiltrate and sabotage enemy bases, or request your help to solve complex mysteries and investigations.  Side Missions are shorter tasks like gathering items or killing small groups of enemies and can be found scattered throughout The Secret World.

Most missions can be completed in any order, but some missions may have prerequisite missions.

A single mission is normally divided into tiers that mark certain levels of progress.  This allows a character to pause a mission and return to it later without losing significant progress.   

If you try to take a new mission but already have a mission of that type, you'll receive a notification asking you to either "Accept" the new mission and pause the current mission, or "Cancel" and keep your current mission.  Resume the paused mission by returning to the NPC who gave the mission originally or by having another player share it with you.  Since you can have up to 3 Side Missions, you will only be prompted to pause one if you try to take a side mission while you have 3 other Side Missions active, and you can select which of your current 3 side missions to pause in the notification window that appears.