You use exterior to refer to the outside parts of something or things that are outside something. The exterior walls were made of pre-formed concrete. = outer, outside
The exterior of something is its outside surface. In one ad the viewer scarcely sees the car's exterior The exterior of the building was elegant and graceful. = outside
You can refer to someone's usual appearance or behaviour as their exterior, especially when it is very different from their real character. According to Mandy, Pat's tough exterior hides a shy and sensitive soul. = facade