If you want to know the actual name for a bodystyle, rather than what marketing determined them to be, this is how it works:

Coupe - a two door vehicle with a sleek roofline. Hence, a '68 Road Runner with a B-pillar is a coupe. A hardtop, a coupe without a B-pillar, would look the same.

Hardtop - see above.

Sedan - a vehicle with a boxier roofline and a B-pillar. Can be a 2-door or a 4-door. A '67 Belvedere I post coupe is really a 2-door sedan. There are also hardtops in this style, such as the 4-door hardtop sedan that first came out in 1956 with some GM models; when it comes to Mopars in the muscle car era, only C-bodies had 4-door hardtop sedans.

Still confused?