A Sedan comes in a variety of styles. There is the notchback, fastback, hardtop and Hatchback sedans. These all have a few of the same characteristics. They all have a full height roof that continues all the way to the rear window. Also, atleast traditionally in the United States they normally have fixed window frames, opposed to the hardtop style of vehicle with no window frames.
Notchbacks have a very distinctive passenger area, and generally the roof is flat and stays parallel to the main body of the vehicle. Once the roof reaches the rear window, the roof line then drops at a steep angle to end directly in front of the trunk area. Ex: Chevrolet Caprice or Ford Crown Victoria
Fastbacks tend to have a continous decline built into the roof and rear window, making it look like a fast back, going all the way into the trunk. Most of the time incorporating the trunk into the design. Ex: Porsche Panamera or 40′s Chevrolet 4-doors.
Hardtop Sedans are similar to the hardtops, but have a pillar in between the front and back doors that go from the floor to the roof, while a normal hardtop has no full pillar, and ends at the top of the doors. Ex: Chrysler 5th Ave or 1971 Ford Thunderbird.