Full Round Action
A Full Round Action is typically an action that takes place on your turn. This also sacrifices all remaining actions in the round and often offers a bonus based on the number of actions sacrificed.
A Standard Action is typically an action that takes place on your turn and uses only one action.
(Example: Punching Someone)
An Instant Action is typically an action which can be performed AT ANY TIME even when it is not your turn. Using an Instant Action does cost you a Standard Action which is typically taken from the bottom of your pile (your last available action)
(Example: Most Defense attempts)
A Free Action is typically an action which can be performed AT ANY TIME like an Instant Action. The difference between an Instant Action and a Free Action is that it does not require the expenditure of one of your actions for the round.