Phil Zimmermann designed Pretty Good Privacy (PGP) for email encryption in 1991.
Zimmermann created PGP to give ordinary people access to strong cryptography for protecting messages and files. The first release combined public-key encryption, which solved the problem of securely exchanging keys, with symmetric encryption for efficiently protecting the actual data. It also supported digital signatures, allowing recipients to verify who sent a message and whether it had been altered.
PGP spread rapidly through bulletin-board systems and the early Internet because Zimmermann released it as freeware with its source code. Its distribution attracted attention from civil-liberties advocates, dissidents, and cypherpunks. The U.S. government investigated Zimmermann over alleged violations of encryption-export rules, but the investigation ended without criminal charges in 1996.
A common mix-up is confusing PGP with TLS or SSH. TLS protects network connections, while SSH secures remote logins and related sessions. PGP protects the message itself, so an encrypted email can remain protected even after it has been stored or forwarded.