A SEMI is a type of vehicle with many axles. It is also known as a SEMI-trailer truck, tractor-trailer, or an articulated lorry. It is used to transport goods over long distances and is composed of a tractor unit and one or more trailers. The tractor unit is the engine and cab of the truck, while the trailer is the cargo area. The number of axles on a SEMI can vary, but typically there are four or more.