PGP (Pretty Good Privacy) is an encryption program, and its encryption format which eventually became the OpenPGP standard.
Created by Phil Zimmerman in 1991, it got into some legal trouble early on because encryption of this grade was classified as a munition under U.S. law and restricted from export. These laws, while not completely repealed even now, have been considerably liberalized since, and now export of PGP and related software is only restricted when it's being exported to countries such as Iran that are under special sanction. Zimmerman's PGP company was eventually acquired by Network Associates (now McAfee), which eventually rebranded the original command-line-based PGP as "McAfee E-Business Server" and sold off all other PGP assets to some PGP developers who formed a new PGP company, which was eventually acquired by Symantec. Meanwhile, the specifications of the file format had been released as the OpenPGP spec and published as an RFC document, making it available for use in other products.
GNU Privacy Guard (GnuPG, GPG) is a commonly-used encryption tool using PGP format, running on Linux systems.
PGP uses a combination of several encryption techniques applied serially, including symmetric-key and public-key cryptography.