Check out this page for more information: Reese Hitches

You'll need to purchase a hitch ball for the receiver hitch. There are different size balls and the bar itself can be angled so that a higher vehicle can tow a towed vehicle that has a lower tow bar. 

Receiver Hitch and Tow Hitch Ball

The Tow hitch is the square hole found on the back of the tow vehicle (Vehicle that is going to do the pulling).                

The Trailer Tongue - This is the part of the trailer that is usually a triangular shape. At the end there is a round coupler that fits over the hitch ball.

The Hitch ball - This is attached to the ball mount.

The Receiver - This is the square receptacle part of the trailer hitch that the ball mount will slot into.

You must match the weight of your towed vehicle to a hitch ball class:

Class I (to 2000 lbs) and Class II (to 3500 lbs) are for light loads. The receiver is a square hole 1.25 inch x 1.25 inch.

Class III (to 5000 lbs) and Class IV (to 10,000 lbs) can accommodate much larger trailers such as campers, boats, etc. A Class III/IV receiver is 2 inches square

You will need a hitch ball with a load rating equal to or exceeding the Trailer Gross Weight Rating.

A hitch ball has 3 measurements you need to know to in addition to the GTWR (refer to image):

A: Ball Diameter (inches across hitch ball)
B: Shank Diameter (inches wide of the threaded shank)
C: Shank Length (inches tall from top to bottom of shank)

The Center of Dot Com