ASP.Net uses windows authentication along with IIS authentication. IIS in turn has different ways to authenticate; anonymous, basic, digest, windows integrated (NTLM & Kerberos) & client certificates.
Takes inputs from the user through a webform and then authenticates them against a DB table or text file etc to verify if the user is valid.
It’s a centralized authentication service provided by Microsoft. User has to login\register only once and then the information can be used by the member sites or subscribing applications to verify the user.