Pretty Good Privacy
(n.) Abbreviated as PGP, a technique developed by Philip Zimmerman for encrypting messages. PGP is one of the most common ways to protect messages on the Internet because it is effective, easy to use, and free. PGP is based on the public-key method, which uses two keys -- one is a public key that you disseminate to anyone from whom you want to receive a message. The other is a private key that you use to decrypt messages that you receive.
To encrypt a message using PGP, you need the PGP encryption package, which is available for free from a number of sources. The official repository is at the Massachusetts Institute of Technology.
PGP is such an effective encryption tool that the U.S. government actually brought a lawsuit against Zimmerman for putting it in the public domain and hence making it available to enemies of the U.S. After a public outcry, the U.S. lawsuit was dropped, but it is still illegal to use PGP in many other countries.