For some, perhaps the real question is Why XML? (or What is XML?), but assuming you have answered those questions (and are using XML), then the next step is to locate an appropriate data model for your content. Deciding on the proper data model is an important step because you will spend lots of time and money developing processes and selecting tools to support your chosen data model. XML, by definition, is extensible and allows you to create any valid structure that suits your needs, but before you go off and develop your own, it might make sense to evaluate the pre-existing options. If you can leverage and build on top of someone else's work, why not?

DITA is a data model for authoring and publishing topic-based content. It was developed by IBM for internal use and has since been released to the open-source community (now under the guidance of OASIS). This architecture and DTD were designed by a cross-company workgroup representing user assistance teams from across IBM. After an initial investigation in late 1999, the workgroup developed the architecture collaboratively during 2000 through postings to a database and weekly teleconferences. Since that time IBM has migrated thousands of pages of content to DITA.

But, Why DITA? Well, assuming your content fits into the topic-based data model, DITA's increasing popuparity means that more and more authoring and publishing tools will be developed to support that model. The DITA Open Toolkit allows you to generate many popular output formats (HTML, HTML Help, PDF, Java Help, etc.) from DITA-based content. If you develop your own data model, you'll have to pay to develop those transformations. Through DITA's modular architecture, it supports efficient reuse of content at the word, phrase or topic level. DITA also has the concept of "specialization," which allows you to develop elements of your own that are based on core DITA elements. This helps you to customize DITA to support your particular types of content.

The following articles provide additional information:

• Going DITA - by Constantine Hondros Oct. 12, 2005 03:21 PM
  It's hard to go to a content management or publishing technology conference these days without there being a presentation on DITA — the Darwinian Information Typing Architecture. For the uninitiated, DITA is an XML architecture for authoring and publishing topic-based content, typically technical documentation. The brainchild of IBM, where it is used internally for many documentation projects, DITA is now an open-source standard under the aegis of OASIS. A reference implementation containing a toolkit is available from Sourceforge.
• The Holy Grail of Content Reuse: IBM's DITA XML - Cover Pages, April 25, 2003
  Researchers at several IBM labs are collaborating on enhancements to the DITA XML system. The Darwin Information Typing Architecture (DITA) is "an XML architecture for designing, writing, managing, and publishing technical documentation, whether in print, as online help, or on the Web. It implements the principles of information design, information typing, and information architecture."
• Why use DITA to produce HTML deliverables? - IBM developerWorks, 07 Oct 2003, Updated 28 Sep 2005
  The Darwin Information Typing Architecture (DITA) is an XML-based format for structuring and authoring technical content. This article explores advantages DITA provides for producing HTML content -- including easy global changes, portability through standards, superior linking and Web management, conditional processing, content and design reuse, and better writing through focused content. DITA consolidates all of the benefits in a consistent, overall information architecture that can evolve and grow along with your product information needs and delivery modes, and with the evolution of standard tools for delivering XML as the presentation mechanism.
• Introduction to the Darwin Information Typing Architecture - IBM developerWorks, 01 Mar 2001, Updated 28 Sep 2005
  The Darwin Information Typing Architecture (DITA) is an XML-based, end-to-end architecture for authoring, producing, and delivering technical information. This architecture consists of a set of design principles for creating "information-typed" modules at a topic level and for using that content in delivery modes such as online help and product support portals on the Web. This document is a roadmap for DITA: what it is and how it applies to technical documentation.
• DITA webinar, "What You Should Know About DITA" - Arbortext, April 28, 2004
  DITA Q&A .. What is DITA?; What’s so great about DITA?; Does DITA allow for use of a custom DTD?; How much of the flexibility and reuse demonstrated during the presentation was due to DITA and how much simply to having XML source?; and more.