There are two kinds of encryption (as far as this discussion is concerned, anyway). First is what is called "symetrical" encryption-this has both parties using the same key to encrypt and decrypt messages. There are a few products that will do this, but it has a significant flaw: you have to get the key from one party to the other secretly. That's hard to do if you're in Miami and your correspondent is in Vancouver.
Asymetrical encryption is different. With asymetrical encryption, also called "public key encryption," each user has two keys-an encryption key that can be distributed to anyone through any means, since it can only be used to ENcrypt data, and a decryption key that is kept secret and doesn't need to go anywhere. PGP, GPG, and a few other products use this, and it's superior because there's no key distribution issue. You email your "public" encryption key to your correspondent in the clear, and your correspondent sends you his the same way. Sure, anyone may be able to grab that key, but the ONLY thing they can do with it is encrypt data that only the owner can decrypt.
I really like PGP because it's been around for a long time and is really cryptographically secure. The author has always made the algorithm available for evaluation by people who can actually tell whether it's easily broken or not, and it's always been nice and strong. Contrast that to some other schemes that are kept secret. A secret algorithm may be just fine-but why keep it secret if you're sure of its strength?
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