A Prone creature is one that has been knocked down or is otherwise laying on the ground, such as a creature under the effect of Sleep. However, a creature suffering the effects of being Stunned or paralyzed are also considered Prone.
Prone creatures are considered Flat-footed and therefore lose their Dexterity bonus to AC. Creatures attacking a prone individual with a melee weapon get a +4 to their Attack Roll. However, characters attacking a prone individual with a ranged weapon get a penalty of -4 to their Attack Roll. Also, getting up from a Prone position incurs an Attack of Opportunity, but recovering from being Stunned or Paralyzed does not.