ASP.NET 3.5 provides to use Windows and Form authentication to our application. From this post we can see how to use Windows authentication in our application.
Authentication is the process of identifying and validating the client accessing your application. Simpler, it is the process of identifying “Who” the end user is and when they visit the site. Authentication is typically used in combination with Authorization — which is the process of figuring out whether the authenticated user has permissions to access a particular page/resource or to perform some action. In ASP.NET terms, authentication is the process that determines the identity of a user. After a user has been authenticated, a developer can determine if the identified user has authorization to proceed. It is impossible to give an entity authorization if no authentication process has been applied. You should never authorize access to resources you mean to be secure if you have not applied an authentication process to the requests for the resources