A swift action consumes a very small amount of time, but represents a larger expenditure of effort and energy than a free action. You can perform one swift action per turn without affecting your ability to perform other actions. In that regard, a swift action is like a free action. However, you can perform only a single swift action per turn, regardless of what other actions you take. You can take a swift action any time you would normally be allowed to take a free action. Swift actions usually involve spellcasting or the activation of magic items; many characters (especially those who don't cast spells) never have an opportunity to take a swift action.
Casting a quickened spell is a swift action (instead of a free action, as stated in the Quicken Spell feat description in the Player's Handbook). In addition, casting any spell with a casting time of 1 swift action (such as choose destiny) is a swift action.
Casting a spell with a casting time of 1 swift action does not provoke attacks of opportunity.