For detailed background on cookies you may wish to refer to the Wikipedia entry on the subject of Cookies

This web site uses a cookie to validate your login credentials. It stores a small string of encrypted text on your system that can only be red by this web site and is used to ensure the site knows who you are and that you have successfully been authenticated.

No other information is stored and it is only stored on your system - any security or privacy concerns you may have heard about associated with cookies are not related to this type of cookie but those used to track your movements across web sites - this web site cookie do not do that - it is set only when you log in and then erased if you log out.

For the technically minded:
This sites uses a encrypted hash stored in a domain-specific session cookie unless the user selected the "remember me" box in which case it is a persistent cookie with a one year timeout.