A DTD file defines the format of an XML file, in terms of expected elements, how they are nested and repeated, and what attributes they have. In the OpenStreetMap world we use XML for various things, so it may be useful to define XML structures with a DTD file. We might also use XSDs
.osm format / map call

The XML we might be most interested in defining in this way, is the .osm file format, and responses from the API 'map' call (more or less the same XML format)

    API v0.6/DTD The structure for the map call in API v0.6 (added notably the user, preferences, gpx_file, and changeset elements, and their associated queries)
    API v0.5/DTD The structure for the map call in API v0.5 (as it was up until early 2009)

source: http://wiki.openstreetmap.org/wiki/OSM_XML/DTD