A web service essentially provides the capability of RPC (Remote Procedure Calling) on top of the protocol of the web (HTTP). A URL implements an API which accepts a set of function arguments and returns a value. Different approaches are used to implement this RPC mechanism on top of HTTP protocol. XML-RPC defines a simple mechanism to specify the arguments and response using XML. SOAP is a much advanced version of XML-RPC. JSON-RPC lets you specify the procedure arguments and return values using JSON (JavaScript Object Notation).

Several programming languages come with built-in support for developing and working with web services. For example in Python, xmlrpclib provides the client side functionality of XML-RPC protocol. The XmlRpcServer library in Python makes it very easy to develop an XML-RPC based web server. Web services are interoperable in the sense that the client and server can be easily implemented in different programming languages and they don t need to worry about the details of each other.

Web services are different from other RPC mechanisms like COM/CORBA/JAVA s RMI. These RPC mechanism use binary data for exchange of arguments and results. The web services uses text oriented protocols like XML/JSON to implement the RPC protocol. Hence they are heavier from the perspective of communication overhead. They are still very good for development of loosely coupled systems. One big advantage they have is the fact that they are not tied to a specific programming language.

By "web service" one normally refers to a service available through HTTP protocol. The protocol sitting on top of HTTP can vary (XML, SOAP, JSON etc.).

But of course one can "complicate" the definition at will :-)

This is example of web service: http://www.webservicex.net/CurrencyConvertor.asmx?op=ConversionRate

When you scroll down you will see you can supply two parameters and get result back. Webservices can support different methods of communication. You can communicate with it using XML, SOAP, or HTTP GET/POST.

So web-service is a special type of script which main function is to expose method to public use (you can set access permissions a well), perform calculations on server-side and return output.