mod_auth_cache transparently caches an authentication originally done by a different module. The aim of this module is to speed up complex authentication mechanisms, to reduce database traffic if authenticating thru a backend database, and to implement some sort of authentication reliability if the database server fails. As the caching is completely transparent (done by the webserver), you don't have to change or implement new functions in your existing applications. The module only caches a users authentication and not a users access permissions, which can still be controlled by a different module.
