A map is a file that contains a hierarchical list of links to topics (topic references or topicrefs), something like the bookmarks menu in your web browser. A map represents the table of contents of a document, or a section of a document. When a map is published, the publishing process creates the final output by pulling together the topics that the map points to.
Maps can also contain references to other maps (map references or maprefs). For example, the map for a document can contain references to a different map for each section.
A bookmap is a special type of map that provides more control over book-oriented features, such as front matter, back matter, chapters, table of contents, appendices, and metadata.
Map files have a .ditamap extension, and are often referred to as DITA maps.