Authentication modules are used to implement different kind of http authentication.

The "basic" authentication scheme is based on the model that the client must authenticate itself with a user-ID and a password for each realm. The realm value should be considered an opaque string which can only be compared for equality with other realms on that server. The server will service the request only if it can validate the user-ID and password for the protection space of the Request-URI. There are no optional authentication parameters.

Implements HTTP Digest authentication. It's more secure than Basic auth since password is encrypted with a "key" from the server.

Delegate used to let authentication modules authenticate the username and password.

Let's you decide on a system level if authentication is requried. You can also decide if authentication is required in each HttpModule.