This is a Class I 1-1/4" hitch and receiver. It attachs to the frame near the bumper at two points.
This is a Class II 1-1/4" hitch and receiver. It attaches to the frame near the bumper at two points and is also secured at another two points further along the frame.
This is a Class III 2" hitch and receiver. Notice that it attaches at six locations and spreads the load out over a larger area.
This is a Class IV 2" hitch and receiver. You need a lot of frame to support this one.