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The following architecture is the foundation of web services:
A service provider is a company or person that builds a web service. They intend to create a service that is widely being adopted. Therefore, they publish their service at a service registry. A service registry is nothing else than a big library for looking up web services. If you want to call an existing web service, you become the service requestor. You look for a web service that matches your needs in the service registry. Once you find a matching service, you want to call its functions, using your own piece of software. To do that, you bind yourself to this service provider, by following the specific communication protocol.
A good example of such a service registry is programmable web (http://www.programmableweb.com/apis/directory/).
Please go to this link and take a look around. There are more than 10.000 web services to explore.
Building and using web services gives you certain benefits. There exists is a strict separation of responsibility. A web service is built for one purpose only. This is the foundation of component Service Oriented Architecture (SOA). Multiple smaller components are easier to manage than one big component. Furthermore, because the communication happens over a protocol, it does not matter what programming language the other party is running. The only thing you know of a web service is how to communicate with it, using the protocol. We will go into details on two of these protocols in the next two chapters.
Here are a couple examples of web services:
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