The creature is lying down. However, if the creature is climbing or flying, it falls.
     The only way the creature can move is by crawling, teleporting, or being pulled, pushed, or slid.
     The creature takes a -2 penalty to attack rolls.
     The creature grants combat advantage to attackers making melee attacks against it, but it gains a +2 bonus to all defenses against ranged attacks from attackers that aren’t adjacent to it.

A creature can end this condition on itself by standing up. A creature can drop prone as a minor action.
    This condition can affect limbless creatures, such as fish and snakes, as well as amorphous creatures, such as oozes. When such a creature falls prone, imagine it is writhing or unsteady, rather than literally lying down. The game effect on that creature is the same as for other creatures.

Published in Player's Handbook, page(s) 277, Player's Handbook 3, page(s) 222, Rules Compendium, page(s) 232, Monster Vault: Threats to the Nentir Vale, page(s) 124.