The various pieces of a tow hitch (also known as a tow bar) are as follows (as seen on cars and non-industrial trucks). A small variety of cars, the most popular kind of automobile. ... The driver of this DAF tractor with an auto-transport semi-trailer prepares to offload Skoda Octavia cars in Cardiff, Wales For further uses of the word truck, see Truck (disambiguation). ...

The hitch, is the receiver which bolts to the chassis of the vehicle. There are a few common classes, I, II, and III. Class I and II are for light loads, and the receiver is a square hole 1 1/4" x 1 1/4". Class III is a "real" hitch and can accommodate much larger trailers such as campers, boats, etc. A Class III receiver is 2"x2".

The trailer tongue slips over a ball. Balls come in various sizes (1_7/8, 2, and 2_5/16. *Note these measurements are in inches), depending on the load they carry. The load of the tongue on the ball is commonly specified as the trailer's tongue weight. Proper trailer loading avoids making this too high, lest steering be affected, or too low, which could involve traction loss.

The ball attaches to a ballmount. You must match the ballmount to the correct hitch class. The ballmount is merely a rectangular bar that may or may not drop down to get the ball closer to the ground so the trailer will ride level.

Sources

Carlist trailer pointer.