There are three authentication modes are available in .net

1. Windows :

It allows web application to authenticate remote or local users based on their windows credentials. Its a Default one.

2. Forms :

It allows web application to authenticate user credentials gathered through a custom login form.

once the user is authenticated (thro username and password) via his login form, the page he requested originally is displayed and a session cookie with the default timeout is stored in the user's computer.

3. Passport :

    Microsoft passport is a centralized authentication service that sites can register and use at a fee. For a web appliation to use the passport service the passport SDK must be downloadd and installed on the web server.

This configuration entry in the web application's web.config file will enable authentication through the passport provider.