Скачать или смотреть How to: Implement Simple Forms Authentication part1

Forms authentication lets you authenticate users by using your own code and then maintain an authentication token in a cookie or in the page URL. Forms authentication participates in the ASP.NET page life cycle through the FormsAuthenticationModule class. You can access forms authentication information and capabilities through the FormsAuthentication class.
To use forms authentication, you create a login page that collects credentials from the user and that includes code to authenticate the credentials. Typically you configure the application to redirect requests to the login page when users try to access a protected resource, such as a page that requires authentication. If the user's credentials are valid, you can call methods of the FormsAuthentication class to redirect the request back to the originally requested resource with an appropriate authentication ticket (cookie). If you do not want the redirection, you can just get the forms authentication cookie or set it. On subsequent requests, the user's browser passes the authentication cookie with the request, which then bypasses the login page.
You configure forms authentication by using the authentication configuration element. In the simplest case, you have a login page. In the configuration file, you specify a URL to redirect unauthenticated requests to the login page. You then define valid credentials, either in the Web.config file or in a separate file. The following example shows a section from a configuration file that specifies a login page and authentication credentials for the Authenticate method. The passwords have been encrypted by using the HashPasswordForStoringInConfigFile method.