As the adventurers explore the world and interact with its inhabitants, game play is usually free-form, guided by the roleplaying of the players and the DM. When the adventurers enter combat, though, the game becomes very structured.
The creatures involved in combat each take a turn over the course of a round. A creature can take a limited number of actions each round, and each action has a type. During some rounds, a creature spends its entire allotment of actions, and during other rounds, it might not take a single action.
A creature gets the following three actions on its own turn.
Standard Action: A standard action requires more effort than any other type of action and is usually the main action of a creature’s turn. Making an attack almost always requires a standard action.
Move Action: A move action involves movement from one place to another.
Minor Action: A minor action involves a simple activity of some kind, such as opening a door or picking up an item.
A creature’s allotment of actions includes some actions that it can take on others’ turns.
Immediate Action: An immediate action is always in response to a trigger on another creature’s turn (such as an action or an event), and either interrupts the trigger or reacts to it. A creature can take only one immediate action per round.
Opportunity Action: An opportunity action is like an immediate action, but it always interrupts its trigger. Also, a creature can take a single opportunity action on each turn except its own.
The one type of action that is rarely limited is the appropriately named free action.
Free Action: A creature can take free actions on its own or anyone else’s turn. Because most free actions require at least a small amount of time, the DM can restrict the number of free actions a creature can take during a round.
Published in Rules Compendium, page(s) 28.