A class 8 truck has a gross vehicle weight over 33,000 pounds.
A class 7 truck has a gross vehicle weight from 20,xxx to 33,000 pounds.
Class 8 trucks usually have larger cabs and a wider selection of engines, drive trains and axles. Most class 8 models can be spec'd with an O.E.M. sleeper.
Class 7 trucks usually have smaller cabs and have limited options for engines, drive trains and axles. Sleepers for most class 7 trucks come from after market suppliers.
An analogy can be drawn between class 7 and 8 trucks and 1/2 ton and 1 ton pickup trucks. A 1 ton pickup is designed for heavy duty applications. It can haul more weight and has more engine options than a 1/2 ton.
The letter designations used by expedite carriers refer to weight carrying capabiltiy and box size.
C unit- usually a 14-16 foot box and able to legally carry 3000 - 5000+ pounds.
D unit- usually a 20-24 foot box and able to leagally carry 12,000 to 13,000+ pounds.
These specifications vary from carrier to carrier. What I have listed are very general specs.