GnuPG
E-Mail Encryption
GnuPG is a software which implements a public-key algorithm for encryption and authentification of data. Every participant creates a private key, which is kept secret, and a public key, which is distributed to other people. With this public key, one can encrypt a message, which can only be made readable with the secret key.
My Public Key
If you want to send me an encrypted message, or verify a signature I made, you can do this with my public key.