XML stands for eXtensible Mark up Language. Despite its name it arguably isn't a language at all. It might be more helpful to think of it as a metalanguage; a set of rules for creating your own language.
It was created by the World Wide Web Consortium (W3C). The idea was to create a way for people to make their own mark up language. Thus giving them much greater freedom to use data effectively by allowing them to create mark up languages for their own specific purposes.
Document Type Definitions are central to the process. They are used to tell other people about the rules that apply to a particular XML document. This allows them to know exactly what that document is doing. It also opens up the possibility that they could adapt that piece of XML for their own purposes.
The Use of XML in Healthcare Informatics
Healthcare Informatics is an important growth area for Librarians and other Information Professionals. Broumphrey and Harrison have investigated the use of XML in this field. They believe that it will help healthcare services to deal with patients information much more effectively. They claim that, "XML has taken the business world by storm .... it is likely that it will have a similar effect on the healthcare industry." (Broumphrey and Harrison, 2000).