A valid driver's license is required for all persons who wish to operate a motor vehicle. Driving without a valid driver's license is a violation of Vehicle Code Section 12500(a). This offense can be either a misdemeanor criminal offense or a traffic infraction.
Vehicle Code Section 12500(a) states as follows: "A person may not drive a motor vehicle upon a highway, unless the person then holds a valid driver's license issued under this code..." Most drivers possess a "Class C" license. This allows a person to operate any two axel vehicle weighing less than 26,000 pounds. This covers most vehicles that you see driving on public roads and highways. A Class C license also allows a person to driver a three axel vehicle with a weight less than 6,000 pounds, a housecar 40 feet or less, and to tow one vehicle.
Other types of driver's licenses are required for different types of vehicles. For example, you must possess a "Class M" license to operate a motorcycle. There are several different types of licenses that may be required of commercial drivers. The specific requirement will depend on the type of vehicle and/or what they are transporting. For example, a "Class A" commercial driver's license is required for any combination of vehicles with a gross vehicle weight rating of 26,001 or more pounds, provided the gross vehicle weight rating of the vehicle being towed is over 10,000 pounds.
If you are cited for driving a vehicle without the proper license for that vehicle, e.g. you have a regular "Class C" license but are cited for speeding while driving a tractor trailer, which requires a "Class A" license, you will be cited for driving without a license and face a possible misdemeanor criminal charge.