Stunned creatures stand helplessly in a motionless stupor. This is considered a mind-affecting effect (unless it originates from stunning fist). Stunned creatures are considered flat-footed, and creatures attacking them receive a +2 bonus to their attack rolls. Being stunned interferes with being prone and is overridden by paralysis, sleep, and petrification.
If a creature is both stunned and prone, the associated modifiers to an attacker's roll stack, but the prone condition is removed after a single flurry. (For example, if a stunned creature is knocked down, then each blow thereafter during that flurry will receive both the prone modifier and the stunned bonus, while attacks after that flurry will receive only the stunned bonus.)