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I am trying to understand the Pretty Good Privacy algorithm:: and have atteched a pdf about it.

A Diffie-Hellman key consists of four values: p, a large prime, g, a primitive element under mod p, d, a random number between 1 and p-2, and e, which is g^d mod p.

Can anyone break this down into simple words. as to what
p,g,d and e are? p i understand is a large prime i.e. a number that cant be divided by any number except 1 and itself. d is fine a random number, i understand that.
g---? whats "primitive"?

You should read up on these:

http://en.wikipedia.org/wiki/RSA
http://en.wikipedia.org/wiki/Diffie-Hellman