Also known as
GML. Geography Markup Language is a set of
XML components designed to
encode geospatial data. It is another way to store and retrieve
GIS data in a
flat file format. For example, GML files can be used for describing the
boundaries of your school, the path you
recorded with your
GPS on a road trip along the California Coastline, or the
topography of a mountain.
XML is a great way to provide an extensible format for holding meta-data, which is probably why the XML based RSS blogs have been so successful.
Other forms of GIS data formats are Autodesk’s Data Interchange File (DXF), Digital Line Graphs (DLG), Topologically Integrated Geographic Encoding and Referencing Files (TIGER), and GeoTIFF. There are many other GIS formats (see , many of which are proprietary. One of the nice things about GML is that it's an open format that anyone can read, and therefore, likely to be widely adopted by GIS applications. Because it's XML, it can be easily parsed by XML Libraries, which are available in almost every programming language.
Is GML widely adopted yet? Maybe not, but documentation is available and in this programmer's opinion, it will be.
A great example of a GML document is http://www.ccg.leeds.ac.uk/geotools/demos/gml/. The site includes a Java Map Viewer with the example loaded.