Semi trucks are made up of a tractor that does the pulling and a trailer to carry freight. In your search for a used semi truck, consider the types of loads you'll be carrying and how versatile you want your unit to be. If you are an independent contractor picking up loads from various clients, you might want to think about just buying the tractor. On the other hand, if you run a specialized service, such as transporting cars, you will most likely need both tractor and trailer.