PGP is an implementation of several encryption algorithms. They are used to provide authentication and encryption services, mainly for email and file storage. It's non-trivial to use.
Characteristics Uses public key cryptography.
A message that is encrypted with your private key will be able to be decrypted with your public key and vice versa. Automatically compresses messages and puts them into a mailable format. Encrypt a message going to a friend with their public key, and only they can decrypt it, using their private key. A MD5 hash of a message is taken, and encrypted with the sender's private key. The receiver decrypts the signature with the sender's public key, and compares the result of running MD5 themselves on the message with the decrypted result.