Understanding Login Controls in ASP.NET 2.0
The link below explains what are the Login Controls and how they can be used against custom data source instead of SQL Server by creating the custom Membership Provider classes.
By creating the custom membership provider described in the above article, we will still be using the fields that are provided by MembershipUser class (http://msdn2.microsoft.com/en-us/library/system.web.security.membershipuser_members(VS.80).aspx). In order to use extra fields that exists in the custom data source we need to extend MembershipUser class as well.
Hope this helps!!!