I'm going to guess that you're asking what receiver you should get?
If you go to
www.cheaphitches.com they have a good system to help you determine what receiver, and hitch you should get based on your vehicle, and what you intend to tow.
Like Collins suggested you need to evaluate your situation, and depending on what you plan to tow, you may find that you need to get more than just the receiver, and hitch.
FYI, the receiver is the part that bolts underneath the vehicle, and the hitch is the small black/silver/chrome piece that holds the ball, and attaches to the receiver.