What is an Extension of a Web-Service?

Web-Service has a .ASMX extension.Whenever,we create any Web-Service,then by-default it is created with .ASMX extension.
Web-service is used for re-usability purposes meaning that once we create any method inside web-service,then we can access that method from any application or platform i.e. Dot net,Java or any platform. It has a XML format.We can access it only by HTTP protocol.It has a [Webmethod] attribute,which is used for exposing method via SOAP protocol.