Original question: “What is the difference between a ‘truck trailer’ and a ‘semi-trailer’?”
Here is a ‘truck and trailer’. It is simply a rigid (or straight) truck towing a trailer. The trailer has a drawbar which is hooked onto the rear of the truck.
Here is a ‘semi-trailer’. A semi-trailer is a trailer without a front axle. Semi-trailers usually have legs to support them when they are not in use.
A semi-trailer is towed by a ‘road tractor unit’. The front of the semi-trailer sits on a coupling on top of the tractor’s tail.
This ensemble (tractor + semi-trailer) is known as a ‘semi-trailer truck’.
In the UK, a semi-trailer truck is usually called an ‘artic’ because it is articulated. In other words, it bends to go around corners.
Semi-trailers can also be towed behind rigid trucks by using a ‘dolly’ a.k.a. ‘diabolo’. A dolly is a detachable axle assembly with a drawbar and coupling.
Semi-trailers can also be towed on the tail of other semi-trailers. The first semi-trailer has a coupling on top of an extensible chassis, which supports the second semi-trailer.