A Class A and Class C motorhome are pretty similar and built similar to each other.
Class C Motorhomes are somewhere between Class A and Class B and are often built on a truck or van chassis that is specifically designed for a motorhome.
The Class C Motorhome also has an attached cab and most have an overhang that extends over the cab. This area is usually used as sleeping quarters but may also be used for storage.
Class A Motorhomes are built on chassis designed specifically for those motorhomes and are usually larger in size and more box like without the overhang.
The Class A Motorhome is a larger motorhome and can be as long as 45 feet in length and usually require a special license to drive as some have air brakes.