Your stock "hitch" is not a hitch - it's a receiver. And your receiver can be used to add either a weight-carrying hitch or a weight-distributing hitch.
A weight-carrying hitch includes the shank (or ball mount) that slides into the receiver, plus a ball that's bolted to the shank.
A weight-distributing hitch also includes a shank and a ball, but also includes additional parts that make up the weight-distributing capability. The shank usually includes not only a ball mount, but also attachment points where the weight-distributing bars connect.
Here's a picture of a simple ball mount part of a weight-carrying hitch. Add a ball and you have a complete weight-carrying hitch.
Here's a typical adjustable shank used with a weight-distributing hitch:
Here's the complete weight-distributing hitch, including shank, ballmount and ball, and weight-distributing parts. This photo also shows the hitch part of the trailer: