From Authentication Tools for Joomla! (JAuthTools)
JAuthTools is a combination of the old LDAP Tools and Kerberos Tools projects. The combination was made as it was apparent that LDAP Tools needed to be expanded more to fit better (and a minor rewrite) and Kerberos Tools, despite the name, could be effectively used for more than just Kerberos but almost any form of HTTP provided authentication.
LDAP Tools is now designated for Joomla! 1.0.x only with other projects taking over parts of its functionality. Within Joomla! 1.5 there exists a Single Sign In plugin and within the Core Framework there is an LDAP library. The SSO project not only absorbed the eDirectory specific SSO implementation (LDAP SSO) but also absorbed the Kerberos Tools project. Additionally, the HTTP SSO mambot replaced Kerberos Tools in Joomla! 1.0.x and is the prequel to the SSO project (being developed after the fact).
Additionally, Joomla! 1.5 provided the ability to do useful things rapidly with the framework, this lead to the development of the User Sources system to provide for the ability to determine the details of a user from multiple sources. At this time, only LDAP is supported by the system but it is envisaged that in future other options will be used.
For more information, review the Supported Platforms page to see what is available for each system.