Web Services, unlike other ASP.NET files, have .asmx extensions. They are essentially all code with little or no user interface.
Each Web unlike Service can be discovered from a remote server via a .DISCO file, which is on the server containing the Web Service. This .DISCO has all the information necessary to enable the remote server to find out about available Web Services and how to use them.
Web Services make available a lot of information about themselves. They use the service description language (SDL) to do so. Although SDL is what .NET is using now, it is moving to Service Control Language (SCL) very soon. This is a more expressive version of the SDL. It will probably replace SDL by the final release of .NET.