What is a class a b or c truck?

Those are licence classes, not truck classes. Trucks are categorised into eight classes, which are numbered as such:

• Class 1: Gross Vehicle Weight Rating of 0 - 5,000 lbs.
• Class 2: Gross Vehicle Weight Rating of 5,001 to 10,000 lbs.
• Class 3: Gross Vehicle Weight Rating of 10,001 lbs. to 14,000 lbs.
• Class 4: Gross Vehicle Weight Rating of 14,001 to 16,000 lbs.
• Class 5: Gross Vehicle Weight Rating of 16,001 to 19,500 lbs.
• Class 6: Gross Vehicle Weight Rating of 19,501 to 26,000 lbs.
• Class 7: Gross Vehicle Weight Rating of 26,001 to 33,000 lbs.
• Class 8: Gross Vehicle Weight Rating in excess of 33,000 lbs.

Classes 1 - 3 are light duty trucks; 4 - 6 are medium duty trucks; 7 and 8 are heavy duty trucks.

A, B, and C are three classes of commercial drivers licences (CDLs).

Class A: Combinations in excess of 26,000 lbs. Gross Combination Weight Rating (combination of weight ratings of truck and trailer), in with the weight rating of the trailer exceeds 10,000 lbs.

Class B: Single vehicles with a weight rating in excess of 26,000 lbs., or combinations with a Gross Combination Weight Rating in excess of 26,000 lbs, when the trailer does not have a weight rating exceeding 10,000 lbs.

Class C: Vehicle and combinations of 26,000 lbs. GVWR/GCWR or less. These normally would not require a CDL, but require a CDL when they are a: transporting a quantity of hazardous materials which requires placards to be displayed, or b: a bus designed to transport more than 15 persons (including the driver).