Each state has its own method of differentiating between passenger cars and trucks for purposes of registration and licensing. Costs may differ, and trucks aren't always allowed to drive and park as if they were automobiles.
Weight is the primary defining factor for trucks.
A B-licensed truck is rated up to 8,000 pounds. The Illinois Secretary of State considers a vehicle to be a truck if it meets any of four criteria. It must be capable of carrying more than 10 people, set up for use as living quarters, used for pulling freight or used to carry cargo. Ordinarily, the fact that a vehicle is fitted to carry people rather than cargo makes it a car when it's time to visit one of the Secretary of State's 117 licensing facilities. Of the 9.3 million vehicles registered in Illinois, about 6.6 million are licensed as passenger cars and 1.2 million are B-trucks.