27. What is PGP?

This FAQ answer is excerpted from:
PGP(tm) User's Guide Volume I: Essential Topics by Philip Zimmermann
PGP(tm) uses public-key encryption to protect E-mail and data files.
Communicate securely with people you've never met, with no secure channels
needed for prior exchange of keys. PGP is well featured and fast, with
sophisticated key management, digital signatures, data compression, and
good ergonomic design.
Pretty Good(tm) Privacy (PGP), from Phil's Pretty Good Software, is a high
security cryptographic software application for MS-DOS, Unix, VAX/VMS, and
other computers. PGP allows people to exchange files or messages with
privacy, authentication, and convenience. Privacy means that only those
intended to receive a message can read it. Authentication means that
messages that appear to be from a particular person can only have
originated from that person. Convenience means that privacy and
authentication are provided without the hassles of managing keys associated
with conventional cryptographic software. No secure channels are needed to
exchange keys between users, which makes PGP much easier to use. This is
because PGP is based on a powerful new technology called "public key"
cryptography.
PGP combines the convenience of the Rivest-Shamir-Adleman (RSA) public key
cryptosystem with the speed of conventional cryptography, message digests
for digital signatures, data compression before encryption, good ergonomic
design, and sophisticated key management. And PGP performs the public-key
functions faster than most other software implementations. PGP is public
key cryptography for the masses.