The original list Arduinna posted says "explicit" violence, which to me means any violence actually seen on screen (as opposed to violence which you infer from context but isn't seen). That would include PG-13 movie violence, such as Star Trek 2009 and Lord of the Rings-type fighting, since it includes "assault." Whacking someone with a sword or hitting them in the face, even if there's no blood, fits in this definition.

From my memory, the number of vids with "extreme" violence is fairly low.