UberX/XL is regular (unlicensed) drivers for hire. The acceptable range of cars is fairly wide, but I know Crown Victorias (and possibly Grand Marquis) are not allowed to prevent old Taxis & Police Cars from being put into Uber service.

Black/SUV is made up of licensed limo/taxi drivers with a car type requirement, though every time I've used it I've received a Lincoln Town Car.

Uber Plus is being tested in a a few markets (I know LA, for one) which is basically UberX with much tighter car requirements, mostly luxury sedans and SUVs.

Ofcourse, this is just the breakdown in the major US markets... they differ in foreign markets to a wide degree, often dictated by local Taxi/Limo laws.