This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
The main motivation for web services is platform-independent communication over some network.
To be able to accomplish this, data is usually sent in a textual format, for instance XML, JSON etc.
Wikipedia has a decent introduction of web services: http://en.wikipedia.org/wiki/Web_service Best wishes!