An action’s type essentially tells you how long the action takes to perform (within the framework of the 6-second combat round) and how movement is treated. There are six types of actions: Half actions, full-round actions, swift actions, immediate actions, free actions. Not an action, in a normal round.

Each round a character gets two half actions and a swift action by default. Only one spell or spell like ability may be cast per turn if it requires a half action.

A half action allows you to do something, most commonly to make an attack, move up to your speed, cast a spell or perform an action that takes a similar amount of time.

A full-round action generally consumes all your effort during a round. To use a full-round action you must expend 2 half actions

Free actions consume a very small amount of time and effort. You can perform one or more free actions while taking another action normally. However, there are reasonable limits on what you can really do for free, as decided by the GM.

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 only a single swift action per turn.

An immediate action is very similar to a swift action, but can be performed at any time—even if it’s not your turn. This expends your next turn's swift action.

Some activities are so minor that they are not even considered free actions. They literally don’t take any time at all to do and are considered an inherent part of doing something else, such as nocking an arrow as part of an attack with a bow or stopping an activity.