I recommend Google for a visual but in short - skeet is shot in a semi-circle with shooting stations around the perimeter and in the center. The birds come from two houses - set at ends of the semi-circle. The flight of the birds is always fixed - the shooter moves among the shooting positions giving you a variety of going away shots, crossing shots, and approaching shots. With trap, the shooter stays in one position. The thrower is in front of him and throws out birds at random angles and elevations but they are always going away type shots.
5 stand is kind of a cross between the two. I personally like it better.