There's many different Skylines that were sold. All the trim levels had different names for each generation.
The GT-R is the top of the line homologation model. They only sell that car to race. Even though it looks like a Skyline, 99% of the car is entirely its own. There are so many differences between the GT-R and a standard Skyline that it's impossible to list here! (chassis, suspension components, brakes, engine, drivetrain, body panels, etc.) You can just say its an entirely different car altogether.
The rest of the Skyline models either come with a lower displacement RB series engine.
R32 had the RB20 with or without the turbo. Only the non-turbo had an AWD version
The R33 and R34 both had the RB25. The R34's RB25 was an "improved" version that met the new LEV emissions regulations. They also offered an AWD Skyline for those living in Hokkaido.
Also, the only thing that the RB26DETT and the RB25DET shares is the RB name.