Geography Markup Language (GML)
The Geography Markup Language is a standards based XML grammar designed by the Open Geospatial Consortium to describe geographic features. The hope of this grammar is to create an agreed upon method for transmitting geospatial data over the internet.
Like other XML based grammars GML has two pieces required to make it work. The first is a Schema that defines the grammar and all the possible objects generically. The Schema serves more or less as a dictionary that allows a program designed to read GML to correctly interpret a file. The second piece is an instance that contains the actual data to be interpreted.
GML is currently in version 3.2.1, and additional information can be found at http://www.opengeospatial.org/standards/gml .