An action defines when and how often a character can do something.
Generally, each character receives one turn per round, and during each turn, can perform up to one standard, one move, and one minor action. A character may sacrifice each of these three for a lesser action: standard to a move or minor, or move to a minor. This can, for example, allow a character to sustain two effects with a "sustain minor" entry — two minor actions — while still allowing the character to use a standard action to attack in the same turn.
When a character has conjurations or minions, they use the character's actions and generally do not act independently.
Walking allows a character to move a number of squares up to the character's speed. Leaving an enemy's adjacent square may provoke an opportunity attack. To avoid this, a character might shift instead.
Running allows a character to move a number of squares up to the character's speed plus 2. When a character begins running, it grants combat advantage and takes a −5 penalty to attack rolls until the start of his or her next turn.
Shifting allows a character to move one square without provoking opportunity attacks.
Standing up ends the prone condition.
Small actions that tend to let you use your other actions more effectively, escape, using an item, a skill test, among other things. Examples would be marking an enemy or summoning a shaman's spirit companion.
Certain actions are not taken as part of a character's own turn. Instead, they are used when certain conditions are met, that is, when triggered. The two types of triggered actions are opportunity actions and immediate actions.
A character can take up to one opportunity action during each other combatant's turn, and up to one immediate action per round during another combatant's turn. Neither opportunity actions nor immediate actions can be taken during the character's own turn.
Some triggered actions interrupt their triggers. Interruption means the triggered action happens before the trigger, and if the trigger becomes impossible to complete, it is lost. For example, if a monster uses a standard action to make a ranged attack, but an opportunity attack interrupts the monster's attack and drops the monster's hit points to 0, the monster's attack is canceled and its standard action is lost.
Main article: opportunity action
One opportunity action may be taken during each other combatant's turn. An opportunity action interrupts the action that triggers it. The opportunity attack, which allows a melee basic attack against the triggering creature, is an opportunity action available to all characters. Stronger triggered actions are usually immediate actions, as described below, so limited to one per round.
A character may take one immediate action per round, during another combatant's turn. An immediate interrupt interrupts the action that triggers it. An immediate reaction occurs after the action that triggers it. If the trigger is movement, an immediate reaction may occur after the first space of movement, interrupting the rest of the movement, if any.
Free actions take little time or effort on your character's part. Examples would be some simple combat orders or dropping your weapon. Each free action will specify whether it is taken in-turn or out-of-turn, and whether it has a trigger.
Normally, characters may take an unlimited number of free actions to do anything other than use an attack power, but this can be limited at the DM's discretion. In addition, the July 2010 update limited characters to one free action attack power per turn.
The free action attack limit was intended to prevent recursion on certain attack power combinations, but also had an undesired effect. Strictly interpreted, the limit prevented scouts from using dual weapon attack and power strike in the same turn. A subsequent update made power strike no action instead of free action.
Certain powers or effects are flagged as No action. These require even less expenditure than free actions, as they generally don't require expending any effort at all.
Many powers and effects create exceptions to these rules. As always, specific exceptions have priority over general rules.
For example, generally, a basic attack is a standard action, and can't be done on someone else's turn. A warlord can use direct the strike to specifically allow an ally to make a basic attack as a free action. In this case, the specific rule has priority: the ally performs the basic attack during the warlord's turn, as a free action, so it doesn't use up the ally's standard action on his or her own turn.