Web Services are xml based independent and self descriptive application components that are used to communicate with open protocols.
The core of Web Services is a combination of XML and HTTP (XML+HTTP), XML is used as a common language for communication in between different platforms and programming languages and HTTP is commonly used protocol that is used to transfer data between client and server.
Web services are XML-based information exchange systems that use the Internet for direct application-to-application interaction. These systems can include programs, objects, messages, or documents.
To summarize, a complete web service is, therefore, any service that:
1 Is available over the Internet or private (intranet) networks
2 Uses a standardized XML messaging system
3 Is not tied to any one operating system or programming language
4 Is self-describing via a common XML grammar
5 Is discoverable via a simple find mechanism.