In The Prince and that Pauper it mentions The Great Seal, is this really a seal? What is its purpose?

In this book, the search for the Great Seal is one of the
major actions.  It is how they eventually decide that the real prince is telling the
truth about his identity.

At one point in the book, the
seal is described as "a massy golden disk."  Tom talks about how it has "devices and
letters" engraved on it.  So that tells you that it really is a seal.  Back then,
proclamations and letters and such had to be "sealed" to show that they were authentic. 
This meant that wax would be heated and a seal would be pressed into the wax.  That is
what the Great Seal was for -- only the monarchs would have access to the Great Seal so
anything with that imprint must have been approved by them.