What is OpenPGP?
OpenPGP is an open standard for signing and encrypting. It enables RSA or ECC sign/encrypt operations using a private key stored on a smartcard (such as YubiKeys), through common interfaces like PKCS#11.
What is PGP?
Pretty Good Privacy, commonly referred to as PGP, is an encryption program developed by Phil Zimmerman in 1991, originally to enable secure communication between anti-nuclear activists via bulletin board systems (BBSs).
What is GPG?
GPG, also known as GnuPG, is a complete and free implementation of the OpenPGP standard. The YubiKey follows the OpenPGP standard by using the GnuPG program to encrypt and sign your data and communications.
How do these three terms relate to one another?
PGP was created but then acquired shortly after. Because of this, many users were bound by licensing difficulties that PGP Inc. encountered.
OpenPGP was developed so that others could develop their own applications that would interoperate with PGP. It was open standard developed with the aim of making development of software with PGP easier (less bound by licensing issues).
The GNU Privacy Guard, also known as GPG, is a complete and free implementation of the OpenPGP standard, first introduced in 1997. When using the OpenPGP application found on YubiKeys in practice, GPG is most often the tool that will be used.