OpenPGP, PGP and GnuPG / GPG are often used interchangeably - a common mistake.
OpenPGP is technically a proposed standard although it is widely used.
PGP is an acronym for Pretty Good Privacy, a computer program which provides cryptographic privacy and authentication.
GnuPG is an abbreviation for Gnu Privacy Guard, another computer program which provides cryptographic privacy and authentication.
gpg is the name of the binary executable file for GnuPG in Gnu/Linux- and Unix-nased operating systems.
Enigmail is not equivalent to PGP, it implements OpenPGP encryption and digital signatures for the Thunderbird email application by using GnuPG in Ubuntu.
PGP and GnuPG are implementations of the OpenPGP standard. To find out more about those see: