What is a Web service?

A Web service is a unit of application logic that can be accessible via standard protocols. A Web service is defined by the W3C as a software system designed to support interoperable machine-to-machine interaction over a network. Web services are frequently just Web APIs that can be accessed over a network, such as the Internet, and executed on a remote system hosting the requested services. Remote clients use XML for data transport and SOAP for using services.

A Web service must have the following characteristics:

1. It must be registered with a central repository so that the client can look up it for a desired service.
2. It contains a public interface for the client to invoke services.
3. It should use standard protocols for communication.
4. It should be accessible over the Web.
5. It should support loose coupling with distributed systems so that the system running on different technologies can cooperate with each other.

The W3C Web service definition encompasses many different systems, but in common usage, the term refers to clients and servers that communicate over the HTTP protocol used on the Web. Such services tend to fall into one of two camps: Big Web Services and RESTful Web Services. Big Web Services use XML messages that follow the SOAP standard and have been popular with traditional enterprise. RESTful Web services have been regaining popularity, particularly with Internet companies. These also meet the W3C definition, and are often better integrated with HTTP than SOAP-based services.

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