DITA (Darwin Information Typing Architecture) is an open standard XML file format that lets you author content in small topic files and assemble those topics into larger documents. DITA also lets you tag text semantically.
Each DITA topic contains a title and one or more paragraphs of text. When you're authoring a document, you author the actual content within topics. You can tag text either semantically or using style-based tags. An example of a semantic tag is uicontrol, which represents a user-interface control. An example of a style-based tag is bold.
Once you've authored your topics, you assemble them into a document using a DITA map. A map does not contain any content. Instead, a map contains several links to topic files, just like bookmarks to HTML pages. By arranging the links in a hierarchy, you create a structure for your document that will become its table of contents.
Finally, you publish your map to an output format, like PDF or HTML. During the publishing process, the content from your topics is assembled into the hierarchy specified by the map.