Off-the-Record Messaging (OTR) is a crptographic protocol which provides encryption for instant messaging like XMPP. It uses public key cryptography.
OpenPGP is a standard for encrypting and authentication of communication, for signing of emails and other files. It uses public key cryptography.
Public key cryptography
In public key cryptography a private and public key pair is generated. During the private key is kept private, the public key is distributed to communication partners. A public key fingerprint (General form: "aaaa bbbb cccc dddd eeee ffff gggg hhhh iiii jjjj") is a sequence of bytes for identifing a public key.
Extensible Messaging and Presence Protocol (XMPP) is a communication protocol for instant messaging. It offers encrypted communication by OTR.