The C.A.S. Server or Central Authentication Server is a single log-in protocol for Internet use. A CAS Server allows a user to access multiple applications by providing the user login and password credentials just once. Additionally, a CAS Server allows authorized web applications to authenticate specific users while keeping the user’s security credentials private.
More details on CAS:
“The CAS protocol involves at least three parties: a client web browser, the web application requesting authentication, and the CAS server. It may also involve a back-end service, such as a database server, that does not have its own HTTP interface but communicates with a web application.
When the client visits an application desiring to authenticate to it, the application redirects it to CAS. CAS validates the client’s authenticity, usually by checking a username and password against a database.
If the authentication succeeds, CAS returns the client to the application, passing along a service ticket. The application then validates the ticket by contacting CAS over a secure connection and providing its own service identifier and the ticket. CAS then gives the application trusted information about whether a particular user has successfully authenticated.
CAS allows multi-tier authentication via proxy address. A cooperating back-end service, like a database or mail server, can participate in CAS, validating the authenticity of users via information it receives from web applications. Thus, a webmail client and a webmail server can all implement CAS.”
– Source: WikiPedia