As far as I can tell, Swift actions are free actions, but you can only take one Swift action per round, similar to a quickened spell. It still needs to be your turn to use a Swift action, you just get to do it without sacrificing any of your other actions for the round.
For example, you could drop a weapon as a free action, move and draw a new weapon as a move action (if your BAB was +1 or higher), use a Swift action (such as Burst), and then take your standard action to attack of manifest a standard power.
Immediate actions are actions that are free, but can be taken outside of your turn in the initiative order.