Send a secret message over untrusted Internet. Encryption prevents anyone from reading your message. Signing protects your message from modification. Public keys allow you to establish trust without meeting physically.
This article shows how you can use PGP encryption with 'gpg' tool. We'll simulate two users to make it easy to practice. PGP is well known, highly secure standard for encryption.
Alice will send a signed, encrypted message to Tero. This is the most obvious, basic use of PGP.