What is a semi-trailer?

A semi-trailer is a trailer that, when combined with a tractor unit, forms a semi-truck. This vehicle combination is used to transport goods and consists of a multi-axle rear unit (the trailer). Compared to other types of trailers, semi-trailers are relatively inexpensive to produce, with most manufactured in China, Germany, and the United States. Depending on their purpose, there are various models of semi-trailers, some of which can even be adapted for passenger transport.