Big rigs are large, heavy-duty trucks used for transporting goods and materials. They are typically composed of a tractor unit and one or more trailers. The most common type of big rig is the semi, which is a tractor unit with a single trailer. Haulers are similar to SEMIS but have two trailers. Tandems are two trailers connected together and pulled by a single tractor unit. Tractor-trailers are the largest type of big rig, consisting of a tractor unit and two or more trailers connected together.