Pretty God Privacy (PGP) is an application of cryptographic technologies. Using public key encryption, PGP is one of the most widely recognized cryptosystems in the world. PGP has been used to protect the privacy of emails, data, data storage, and other forms of communication such as instant messaging. PGP implements a special kind of Public Key Infrastructure (PKI) trust model called Web of Trust.
Early versions of PGP were written by its creator Philip Zimmermann and first offered to the public in 1991. The program is one example of an open source application and as such has several different versions available.

The terms "OpenPGP", "PGP", and "GnuPG / GPG" are often used interchangeably. This is a common mistake, since they are not the same..
PGP is a computer program which provides cryptographic privacy and authentication.
OpenPGP is technically a proposed standard, although it is widely used. OpenPGP is not a program, and shouldn't be referred to as such. PGP and GnuPG are computer programs that implement the OpenPGP standard.
GnuPG (GPG) is an acronym for GNU Privacy Guard, another computer program which provides cryptographic privacy and authentication.