Hi Kerry,
dehors means outside, as in: Les enfants jouent dehors. -- The kids play outside.
En dehors refers to the outside as a more tangible place and as such has a clear opposite: en dedans. It can also be used figuratively, though.
en dehors -- L'ennemi se trouve en dehors la cité. -- The enemy is outside the city.à l'extérieur -- Il faut repeindre l'extérieur du mur. -- One must paint the outside of the wall.
-- Chris (not a native speaker).