Web Services can convert your application into a Web-application, which can publish its function or message to the rest of the world.
The basic Web Services platform is XML + HTTP.
Basic idea behind web -service is that not just web page but also mobile pages or stand alone application can connect , this also isolate the buisness logic.
For example if we have a server which gives details of available seats on a flight any website can send query in the form of xml or normall SOAP request and get that detail and can show in the format they want.
since XML is used over http for communication we got data independece so a C++ or .NET application can connect to web service , this means write once ,access from anywhere from anything.
there are many application server which provides webservice e.g. websphere , weblogic , iPlanet etc