An overview of the Darwin Information Typing Architecture.
If you need to be especially organized in producing, reviewing, managing, and publishing a large amount of content, DITA is a very useful tool.
DITA allows you to create small chunks of content, and then intelligently organize and reuse those chunks, so you and your team don't end up inefficiently writing, rewriting, or editing the same content repeatedly.
The structure of DITA is facilitated by XML.