A semi-trailer is a type of trailer that is designed to be towed by a truck, known as a tractor. Unlike a full trailer, a semi-trailer has wheels at the back and is supported at the front by the tractor, allowing for easier maneuverability and loading. Semi-trailers are commonly used in freight transport, carrying goods across long distances. They come in various sizes and types, including flatbeds, reefer trailers for temperature-sensitive cargo, and tankers for liquids. This versatility makes them essential in the logistics and transportation industry.