Generally, all of these terms refer to large commercial vehicles. However, there are some distinctions between a few of the commonly-used terms:
A semi-truck is just the tractor or truck part of the unit, where the driver sits and the engine is housed.
A semi-trailer is the trailer that is attached to the semi-truck.
A tractor-trailer is the combination of the semi-truck and trailer.
An 18-wheeler also refers to the combination of the tractor and trailer.
A commercial truck can be a tractor-trailer, but the category also includes other types of trucks used for commercial purposes, including single-unit box trucks or even dump trucks.