Hi! You probably followed the link at the bottom of an email from me and are now wondering what 'gpg' and that block of gibberish at the end of my email are. The short explanation: gpg stands for GNU Privacy Guard and is a cryptographic software package I use to encrypt and digitally sign email. NOTE: This page explains what gpg and pgp do and not how to use gpg.
Obviously, my mail is not encrypted (you can read it, can't you?). Instead, it is protected by a digital signature. To explain this, you need to know that email is very easy to forge: you can write anything you want into the 'From:' header of the email you send, it doesn't need to be your name and email address. Also, mail can be modified on its way from sender to recipient (on purpose or just because of some software problem). A digital signature protects the recipients of my mails from both these problems: everybody who receives an email from me can without doubt confirm that it was me who sent the mail and that nobody tinkered with it, by verifying the email with the gpg or pgp software and with my public key.
But why do I sign my mail? Recently, some efforts are made by governments to regulate the use of 'dangerous' technology (and cryptography is often regarded as one of those). Signing my e-mail on one side is a statement that I do not think such technology should be regulated, and on the other side it's just that I like to play around with this technology. And, of course, just in case somebody really wants to send fake emails that appear to come from me, I'm protected because if an email comes from me, it usually is digitally signed.
To sign or encrypt an email, keys are used - a key is like a password. Every key has two parts: one that I keep as a secret, and another that I publish. In the case of a digital signature on an email, everybody receiving the mail can verify the signature with the public part of my key - and so he can see that the email has not been (maliciously or not) modified and that I was really the author of the email.
The biggest problem with email encryption and signing emails is that (in the case of encryption) the sender has to know that the encryption key really belongs to the person it says, or (in the case of a signature) that the signature was really made by the person it claims (after all, I could just as well claim to be 'Monica Lewinsky', and put that name on my digital signature). Signing a key solves exactly this problem: When somebody signs my key, he confirms that this electronic key really belongs to the person 'Rod Begbie'. The idea of signing one anothers key is called the Web of Trust - this is of course a very short description, a better (and longer) one can be found at http://www.rubin.ch/pgp/weboftrust
There are many sources for more information, a search on google should give some pointers. Some very good documentation is contained in the The GNU Privacy Handbook.