A semi-trailer is a type of trailer that is designed to be towed by a truck, specifically a tractor unit. Unlike a full trailer, a semi-trailer has wheels at the back but is supported at the front by the truck's hitch. This design allows for easier maneuverability and greater cargo capacity, making it a popular choice for transporting goods over long distances. Semi-trailers come in various sizes and configurations, including flatbeds, reefer trailers for temperature-sensitive cargo, and tankers for liquids. They are commonly used in the freight and logistics industry, playing a crucial role in the supply chain by moving products from manufacturers to retailers.