DITA maps are documents that collect references to DITA topics to organize them for output to a specific deliverable, including generating navigation files and links to related topics.
A typical DITA map has a title and a set of topicrefs, in a sequence, in a hierarchy, or both, that define the structure by which DITA topics are to be viewed as a complete information deliverable.
A bookmap is a specialization of DITA map that allows you to organize your DITA topics into a collection that can be printed as a book or other paged layout.
You can find more information about DITA Maps at: http://docs.oasis-open.org/dita/v1.1/OS/archspec/dita_spec_23_maps.html.