The head has a greater diameter than the hole the rivet is inserted into, to act as a fastener on one side of the material being riveted.
Rivet pin
The pin slots through the corresponding holes in the two pieces of material that are being riveted together.
Rivet end
The rivet end eventually becomes another rivet head when it is expanded using force. The rivet end will be hit with a hammer or shaped into a head using a rivet snap.
To find out what a rivet snap is, see How to install a solid rivet.