XML documents by themselves simply are . They don't do anything. To become useful, a document must be processed by some piece of software that interprets the XML structures in some way, such as formatting and displaying it to a person, placing an order with a store, operating a machine, or otherwise interfacing between the computer and the real world. It is in this processing that XML documents achieve meaning and become more than just a sequence of bytes or characters . The information in the XML document stops being purely structural and becomes semantic.
This part discusses the various APIs, tools, and languages available for processing XML. You'll learn how to choose the appropriate APIs and tools for different kinds of jobs. While most of the major APIs can theoretically handle anything you throw at them, in practice some APIs are much better suited to certain types of processing than others are. It pays greatly to have a variety of tools in your toolbox and to pick the right one for the task at hand.