A semi-trailer truck is a large vehicle commonly used for transporting goods over long distances. It consists of a tractor unit, which is the front part that contains the engine and driver's cab, and a semi-trailer, which is the rear part that carries the cargo. The semi-trailer is designed to be attached to the tractor unit, allowing for easy loading and unloading of freight. These trucks are essential in the logistics and transportation industry, as they can carry heavy loads and travel across various terrains. They are often seen on highways and are crucial for delivering products to stores and businesses. The combination of the tractor unit and semi-trailer makes them versatile and efficient for long-haul transportation.