A trailer is an unpowered vehicle towed by a powered vehicle. It is commonly used for the transport of goods and materials. It has wheels but no front axle and no engine, so it can't move on its own. The trailer contains the cargo which is being hauled. The semi-truck also supports a lot of the weight of a trailer when the two are attached. Tractor-trailer and 18-wheeler both refer to the combination of a semi-truck and semi-trailer.