We cannot place the whole world inside our brains, inside our minds, or inside our computers. Many kinds of model can be built of the same thing, and the choices that are made in designing the model will powerfully affect what can be done with it, and whether the results are what we intended or not. Every computer system is built on the basis of implicit or explicit models that define the structure of the data being processed and the information that data represents.
Some models are highly specific, and tailored for a particular purpose.
Other models are quite abstract and generic, and can be used effectively for many different purposes. There are advantages and disadvantages to both the specific and generic modeling approaches, but there are underlying principles of good model design that underpin both. The advent of XML has cast a particular spotlight on the importance of information and data modeling. Because XML is itself based on an extremely generic underlying model, it needs other, more specific models to be developed to apply to the many different uses to which it can be put. The development of these models, which may be expressed as DTDs or as XML Schemas, is an important part of the design and planning of any XML-based application or system.