Authentication is the mechanism whereby one entity proves its identity to another.
In the context of uPortal, we're most often talking about Authentication in the context of a person at a web browser proving his or her identity to the portal. Portals provide personalized, customized, aggregations for particular users. In order to provide this to any given user, we need to know who that user is.
uPortal supports a configurable and pluggable authentication provider mechanism. This is configured via security.properties. Multiple authentication mechanisms can be supported at once, including usernames and passwords stored in uPortal's own database, LDAP authentication, and enterprise single sign on mechanisms such as the Central Authentication Service.