In the most simplistic way a Web service may be defined as "a Web accessable program code which performs a task of either processing or serving some data. Although there are many other definitions in the related literature, the one in W3C (2004) seems to be quite complete and refering to also lately popular REST style Web services. It states that " We can identify two major classes of Web services: REST-compliant Web services, in which the primary purpose of the service is to manipulate XML representations of Web resources using a uniform set of "stateless" operations; and arbitrary Web services, in which the service may expose an arbitrary set of operations.
Update to concept of APIs. Web services are APIs
CHange title to Web services and APIs
identify the web services needed for a particular use case
perform the connection to existing web services to use the resources exposed by the service
select the web services best fit to expose your own resources
understand how different web services complement each other
In progress GI-N2K