Stances are class abilities which generally take a swift action to initiate, can be sustained for free, but are broken when knocked prone or subjected to a condition which reduces your number of available actions for the round. For example, Rattled prevents you from using either a standard or a move action, and thus would end a character's active stance immediately upon being afflicted with it. You can restart the stance with another swift action once the condition has been cleared.
A player character can only have one stance-type ability active at a time.
Stances can only be maintained continuously for 1 minute, though it may be restarted with a new swift action after the duration expires, assuming the character meets any other requirements to re-initiate the stance.
Dropping a stance voluntarily usually costs a swift action. Refer to the stance ability's description for details.