If you want to retrieve user informations you should use the following WebSecurity properties:

• <div style="padding-left: 30px;">WebSecurity.CurrentUserId          Gets the ID for the current user </div>
• <div style="padding-left: 30px;">WebSecurity.CurrentUserName     Gets the user name for the current user</div>

For more informations on WebSecurity see at WebSecurity Class.

On cookies and formsauthentication maybe these links could help you:

Using FormsAuthentication in ASP.NET WebPages

Using FormsAuthentication