One of the best encryption systems is called GPG encryption which is an open-source version of PGP encryption. PGP stand for Pretty Good Privacy and is actually an understatement made by a programmer who didn't want to be too optimistic about how secure it is. However, as it turns out, PGP is has actually proven itself to be extremely good. It's been around for many years, being maintained by the best coders in the world and it hasn't been cracked.
In this Instructable, I'll walk you through the simple process of setting up GPG and then installing a Firefox plugin that will make it easy to encrypt your Gmail.
Step 1: How it works
The two keys work together so that you need both to decrypt anything. To send an encrypted message to someone you lock the message with their public key and when they get it, they can unlock it with their private key. If they want to respond, then they encode the message with your public key and you can read it with your private key.
Of course, this only works so long as you can trust that you have been given the right public key and that you know who you are talking to. One of doing this is by having a key signing party with your close friends. You all show up at a given location at a given time and exchange public keys. Then you have a list of trusted public keys with which you can communicate. This is often referred to as a web of trust.