[Perfectlap]

A Convertible is a car that probably existed as a coupe at one point and to increase sales they decided to hack off the roof. But thats not a strict rule.
Audi A4, S4, TT'; Porsche 911 Cabriolet; BMW 3series, M3 convertible etc.

A roadster is a car designed to be a drop top from the get go. There's a long long history of these cars the British variety stand out the most. Triumphs, Lotus Elan, etc. And recently you Miata, Boxster, S2000 and Z3...all very succesful
Essentially roadster is rwd, two seats, long hood and short wheel base..not very practical.

GT is a grand touring car. Its the Oprah Winfrey of cars, expensive and heavy.
The 911 has essentialy become a GT. They get heavier and heavier and more and more expensive loaded with creature comforts. Not sure why they decided to brand the Carrera GT with the "GT". Its the last car in the world you would want to spend a long drive on wehre you can feel every seam in highway asphalt and the gear shift feels like a Peterbilt. If anything they should have called it the Carrera LMP. That car was never intended to be a street car, which may be the most extreme type of Exotic.

Me thinks a sports car is essentialy a car that is extremely well suited for the track. The more 'civilized' it becomes, (more than two seats, higher and longer stance, more weight,etc.) the less of a sports car it becomes (I think EVO and STI are the rare exceptions). If you visit your average track or autocross you'll notice its predominantly the same 10 or 11 cars no matter where you go, even though the 'sports car world' features dozens, most are pretenders! more luxury than fun.

The more cars I drive these days, some very expensive, the more I realize what an excellent job Porsha did with essentialy the first true roadster in its history. A second hand S2000 and Boxster are excellent, excellent sports cars. You can spend more but you aren't really getting allot of added performance for each dollar.