Kerberos is a standard protocol that provides a means of authenticating entities on a network. It is based on a trusted third-party model. It involves shared secrets and uses symmetric key cryptography.
For more information, refer to RFC 1510.
Simple Authentication and Security Layer (SASL) provides an authentication abstraction layer to applications. It is a framework that authentication modules can be plugged into.
For more information, refer to RFC 2222.
Generic Security Services Application Program Interface (GSSAPI) provides authentication and other security services through a standard set of APIs. It supports different authentication mechanisms. Kerberos v5 is the most common.
For more information on the GSS APIs, refer to RFC 1964.
This SASL-GSSAPI implementation is from section 7.2 of RFC 2222.