The creature can’t take actions.
     The creature grants combat advantage.
     The creature can’t flank.
     The creature falls if it is flying, unless it can hover.

Usually a creature is subjected to this condition because of significant physical or mental trauma. It might be on the brink of succumbing to a deadly poison, completely paralyzed by a ghoul’s bite, reeling under the mental assault of a mind flayer, or battered by an extremely powerful blow.

Published in Player's Handbook, page(s) 277, Player's Handbook 3, page(s) 223, Rules Compendium, page(s) 234, Monster Vault: Threats to the Nentir Vale, page(s) 125.