login controls and Forms authentication

Assuming you're talking ASP.Net, Forms authentication still requires the user to log in.  You can use the built in login controls or make your own.

Once the user is logged in, assuming you're using Forms authentication, the user automatically gets a cookie which identifies him.  Each time the user does something which causes a postback, the cookie gets sent to the server with everything else.  The cookie identifies and authenticates the user.  When the user logs out or his session expires, the cookie will no longer work.