PGP or Pretty Good Privacy
PGP is an acronym for Pretty Good Privacy.
PGP is a program that is used for encryption of documents and files. It was developed by Zimmerman and given to the world for free. The early versions seemed to be freeware. The history of its development and the legal consequences of its release are more than a bit interesting.
This software encrypted a document so it could only be opened by the person it was encrypted for.
It worked with a public key and a private key.
As with any other encryption method the code can be broken if enough man hours are spent attempting to crack the code.
It seems to work quite well and seems to be still available over the internet.