A Web service is a set of functions packaged into a single application programming interface (API) that is available to other systems on a network.
The term Web Service generally refers to clients (consumers) and servers (providers) that communicate over the HTTP protocol used on the Web.
Basically, you can compare the web service logic as a web form process. When you click on the button (ok, send, apply, …) of a web form, you send the data present in the form and the server sends you back other data.
The web service process is the same, you send an HTTP request to a server that sends you back the result not in an HTML format but in an XML or JSON format.
It's why :
The W3C Web service is a key concept of a soa architecture. Application interoperability is the goal of Web services.
For example, a banking Web service may implement functions to:
Such services tend to fall into:
Other approaches with nearly the same functionality as web services are: