Swift and Immediate Actions (As per pg. 4 of Spell Compendium)
Swift Action: A swift action consumes a very small of time, but represents a larger expenditure of effort 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 anytime you would normally be allowed to take a free 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). Casting a spell with a casting time of 1 swift action does not provoke attacks of opportunity.
Immediate Action: Much like a swift action, an immediate action consumes a very small amount of time, but presents a larger expenditure of effort and energy than a free action. However, unlike a swift action, an immediate action consumes a very small effort and energy than a free action. However, unlike a swift action, an immediate action can be performed at any time – even if its not your turn. Casting feather fall is an immediate action (instead of a free action as listed in the spell description in the Player’s Handbook), since the spell can be cast at any time. Using an immediate action on your turn is the same as using a swift action, and counts as your swift action for that turn. You cannot use another immediate action or a swift action until after your next turn if you have used an immediate action when its not currently your turn. You also cannot use an immediate action if you are currently flat-footed.