Forms authentication cookie is nothing but the container for forms authentication ticket. The ticket is passed as the value of the forms authentication cookie with each request and is used by forms authentication, on the server, to identify an authenticated user.
However, if we choose to use cookieless forms authentication, the ticket will be passed in the URL in an encrypted format. Cookieless forms authentication is used because sometimes the client browsers block cookies. This feature is introduced in the Microsoft .NET Framework 2.0