By taking the run action, a creature can use an all-out sprint when it really needs to cover ground quickly. The creature has to lower its guard to make best speed, however, and it can’t attack very well.
Action: Move action.
Speed + 2: The creature moves up to its speed plus 2 additional squares. For instance, if its speed is 6, the creature can move up to 8 squares when it runs.
-5 Penalty to Attack Rolls: The creature takes a -5 penalty to attack rolls until the start of its next turn.
Grant Combat Advantage: As soon as the creature begins running, it grants combat advantage until the start of its next turn.
Despite the action’s name, a creature isn’t always literally running when it takes this action. The action can include other movement modes such as climbing, flying, and swimming.
Published in Player's Handbook, page(s) 291, Rules Compendium, page(s) 248.