In order to use PGP, you'll need to have the recipient's public key. Each person's public key is unique and lets PGP encrypt the message in a way that only the key's owner will be able to decrypt. The nifty thing about public key encryption systems is that a person's public key can be used only for encrypting messages to that person; a separate, private, key is used for decrypting. This means that the public key can really be public: there's no need to try to keep it out of the hands of potential attackers because it will not help them break the code.
PGP was originally written by Phil Zimmermann in 1991. It is the basis for OpenPGP, an internet standard for encrypted email. Commercial and free versions are available; see Phil Zimmermann's Where to Get PGP page for more information.