From Microsoft MSDN:
ASP.NET membership gives you a built-in way to validate and store user credentials. ASP.NET membership therefore helps you manage user authentication in your Web sites. You can use ASP.NET membership with ASP.NET forms authentication by using with the ASP.NET login controls to create a complete system for authenticating users.
ASP.NET membership supports facilities for: * Creating new users and passwords. * Storing membership information (user names, passwords, and supporting data) in Microsoft SQL Server, Active Directory, or an alternative data store. * Authenticating users who visit your site. You can authenticate users programmatically, or you can use the ASP.NET login controls to create a complete authentication system that requires little or no code. * Managing passwords, which includes creating, changing, and resetting them . Depending on membership options you choose, the membership system can also provide an automated password-reset system that takes a user-supplied question and response. * Exposing a unique identification for authenticated users that you can use in your own applications and that also integrates with the ASP.NET personalization and role-management (authorization) systems. * Specifying a custom membership provider, which allows you to substitute your own code to manage membership and maintain membership data in a custom data store.
More detail information please read Microsoft MSDN
Related info: MembershipProvider Class