What is a pin back button?

This is not a button used for fastening a garment. It is made of metal. It attached to your clothing by means of a pin on the back, rather like a safety pin. A pin back button usually has a witty saying, political slogan, or picture on its face. (Other buttons may have a tie-tack clasp: a straight piece of metal that goes through your clothing and has a separate metal piece you put on the back to hold it in place, kind of like a pierced earring.)