A punch is for puncing a hole thru some material. A standard paper punch is a common example. The Die is the part with a hole in it that the phunch goes into after going thru the material.
A paper punch has the punch and the die mounted on opposite sides of the handle set... you can see the punch going into the die.
But that produccess only one size of hole. In most punch and die sets, the dies are in a plate like an anvil... sometimes all the die holes are in the same plate. Then there is a variety of punches to fit the variety of dies.
It is used not only for making precise holes in plastic etc... but also for punching out various sizes of small disks that can be used for detailing.
www.naplak.com/modeling ... a free site for modelers
www.scalehobby.com/forum/index.php ... a nice Modeling Forum