PGP has two uses. First, it is an encryption system that uses public-key cryptography. Each user has a public key and a private key. In simple terms, you can encrypt a message using someone's public key and they can decrypt it using their private key. (A one-off session key is actually involved.) If the private key has been kept truly private, no one else can read the message.
More commonly, PGP is used to create a digital signature based on the contents of an email. This enables the recipient to verify that the message has not been changed, using the sender's public key.
