You could use exterior paint indoors but not vice-versa. The reason being that the outdoor paint is more water and weather resistant. Interior paint would degrade really fast if used on the exterior and fall off the walls in little pieces. The you'd have to clean the exterior and redo everythign from scratch.
You don't want to mix them up. Exterior paint can withstand weathering. Some interior paints are made to be washable, for the bath and kitchen, and are glossier and nicer.
