a coupe is a 2-door hardtop car (there are many types of coupes, you have T-Tops like on a camaro, Targa Top like a supra or Del Sol), a convertible is typically a 2 door with a top that can be put down, and a hatch is pretty much a coup but instead of having a trunk lid seperate from the roof it has a hatch door on the roof connecting to the truck making it all put together with the car. Kinda like most honda civics you see crusing around that look like a wagon but are 2 door.
In addition, a coupe is always a 2 door car. A sedan is always a 4 door car. A convertible can be 2 or 4 doors. (though a 4 door is very rare.) A hatchback is a car with a rear opening to the seats. It has no enclosed trunk and may be 2 or 4 doors.
A sedan is not always 4 doors. It depends on whether or not the door has bracing at the top forthe window to roll up into, or if the window seals against the car. My nova is considered a 2dr sedan.