GnuPG is open-source software that makes it possible to encrypt your email and other messages so that they can only be read by the intended recipient (not unlike Jim Phelps and his briefing book). Unlike cleartext messages, which are unprotected and can be read by anyone who intercepts them, GnuPG-encrypted messages are ‘locked’ through cryptographic techniques and may be viewed only by a person with the correct ‘key’; everyone else will merely see gibberish. In today’s day and age, when hacking and phishing is common, such encryption plays an important role in protecting sensitive data like user names, passwords or credit-card information.
(Via Zend Developer Zone.)