For those too lazy to read the entire Wikipedia article, but still curious, PGP is basically a system for using a Public Key Infrastructure (PKI) to encrypt e-mails, files, etc.
PKI is a category of assymetric encryption algorithms (such as RSA), where there are two encryption keys, a public and a private key.
A public key can only be used to encrypt data. And the corresponding private key can be used to decrypt(but not encrypt) said data. The keys are always in pairs. This allows me to give you just the public key, and you can securely send messages to me. This allows me to practically broadcast my public key without worrying about anything, since the public key can only be used to encrypt. The private key is typically symmetrically encrypted with a pass phrase for extra security, so that even if somebody gets their hands on your private key file, they would still need your pass phrase to decrypt your messages.
Most PKI’s can also be used for secure digital signatures as well.
I hope I didn’t screw anything up.