PGP and similar software follow the OpenPGP standard (RFC 4880) for encrypting and decrypting data.
Pretty Good Privacy (PGP) is an encryption program that provides cryptographic privacy and authentication for data communication. PGP is used for signing, encrypting, and decrypting texts, e-mails, files, directories, and whole disk partitions and to increase the security of e-mail communications. Phil Zimmermann developed PGP in 1991.
Because of PGP encryption's importance worldwide, many wanted to write their own software that would interoperate with PGP 5. Zimmermann became convinced that an open standard for PGP encryption was critical for them and for the cryptographic community as a whole. In July 1997, PGP Inc. proposed to the IETF that there be a standard called OpenPGP. They gave the IETF permission to use the name OpenPGP to describe this new standard as well as any program that supported the standard. The IETF accepted the proposal and started the OpenPGP Working Group. OpenPGP is on the Internet Standards Track and is under active development.
In the free software world, the most popular implementation of OpenPGP is GnuPG. You will often hear people refer to GPG keys instead of OpenPGP keys, these are essentially the same thing.
The EFF have produced guides on how to use GnuPG on: