A GML document is a type of XML document that represents Geographic information.

According to the OGC, the Geography Markup Language (GML) is an XML encoding for the transport and storage of geographic information, including both the spatial and non-spatial properties of geographic features.
This specification defines the XML Schema syntax, mechanisms, and conventions that

• Provide an open, vendor-neutral framework for the definition of geospatial application schemas
  and objects;
• Allow profiles that support proper subsets of GML framework descriptive capabilities;
• Support the description of geospatial application schemas for specialized domains and information communities;
• Enable the creation and maintenance of linked geographic application schemas and datasets;
• Support the storage and transport of application schemas and data sets;
• Increase the ability of organizations to share geographic application schemas and the information they describe.

Implementers may decide to store geographic application schemas and information in GML, or they may decide to convert from some other storage format on demand and use GML only for schema and data transport.

The goals of GML are to provide a means of encoding spatial information for both data transport and data storage, especially in a wide-area Internet context;

• be sufficiently extensible to support a wide variety of spatial tasks, from portrayal to analysis;
• allow for the efficient encoding of geo-spatial geometry (e.g. data compression);
• provide easy-to-understand encodings of spatial information and spatial relationships, including those defined by the OGC Simple Features model;
• be able to readily link and encode spatial (geometric) elements to other spatial or non-spatial elements.
• provide a set common geographic modeling objects to enable interoperability of independently developed applications.

For the current best introduction to GML, check the GML entry on the wikipedia .