I'm no expert, but I will give this a try...
First you pick a "base". This is the pin part. They have many designs....you can see some of them on this page....
pinpics.com/cgi-bin/group.cgi?group=720
Then you purchase "add ons". These are little pieces to put on the "base". They have characters, ride logos, and other Disney things.
Then you take all your pieces and they put them together with the special machine, then you have one pin.
Does that make sense?