Towing locks come in several shapes and sizes, and the more elaborately designed they are, the more secure and harder they'll be to break. That also, of course, makes them more expensive.
The simplest type of towing lock is the swivel lock pin, which takes the place of a standard pin and clip on a trailer hitch receiver. Swivel lock pins are short metal cylinders about 1.25 inches (3.175 centimeters) long and curved slightly at the top. This curve allows the pin to slide through the receiver's connection and have a place to rest, keeping it from falling straight through to the ground. A padlock fits on the straight end of the pin at the bottom after it's put through the receiver, and a unique key locks it in place. Because the design is simple and the materials are relatively inexpensive, swivel lock pins are pretty cheap -- they typically cost between $3 and $12.
A more heavy-duty design is the dog bone-style lock. These types of locks are shaped like dog bones -- the middle section is thin, while the two ends are thicker. One removable end acts as the deadbolt, which snaps onto one end of thin middle section secures your trailer. These are typically made of steel and have greater strength and durability, so they cost a little more at around $30.
Most locks also have some sort of debris protection, typically in the form of a rubber cap that fits over the key slot and protects it from flying rocks and other dangerous materials.
To unlock lots more information on protective towing equipment, see the next page.