A semi-trailer is a type of trailer that is designed to be towed by a tractor unit. Unlike a full trailer, a semi-trailer has wheels at the back and is supported at the front by the tractor, which connects to it via a fifth wheel coupling. This design allows for easier maneuverability and greater cargo capacity. Semi-trailers are commonly used in the transportation industry to haul goods over long distances. They come in various sizes and types, including flatbeds, reefer trailers for temperature-sensitive cargo, and tankers for liquids. Their versatility makes them essential for freight logistics.