Sititing is the non-combat position in which your character crouches or squats on the ground or a chair as opposed to standing.
A character cannot engage in combat or use many items or skills directly from a sitting position, they must first stand. Jumping, walking, or running from a sitting position will cause a character to immediately stand and take the appropriate action.
Melee attacks against a sitting target automatically hit and cause double damage. The first damage taken while sitting will cause a target to immediately stand. The double damage registers as a critical hit in the combat log, but does not trigger any abilities that would normally trigger in response to achieving a crit.
Sitting while in combat immediately stops auto-attacks, but not spell-casting.
It takes a short amount of time to sit from a standing position and about equal amount of time to stand from a sitting position. Some benches or chairs can be right-clicked to put the character into a sitting position on the bench or chair.
Your natural healing rate is increased when sitting.