CAS stands for Central Authentication System. CAS provides a way for web applications to authenticate you as a user. Many web applications can all use CAS for authentication. When you try to access a CAS enabled website, you are redirected to CAS. If CAS determines that you have not entered your username and password for a while, it will prompt you to do so and then redirect you the the website you tried to get to. If you have already entered your username and password in the last few hours, CAS recognizes this and immediately redirects you back to the website that you tried to get to in the first place. In either case, the website you are trying to get to will recognize that you are indeed who you say you are.