XML is a web-based language used to interact data behind the scenes. With projects like GML by the Open GIS Consortium, XML is fast becoming an essential tool for GIS.

ArcXML
ESRI programmer’s refence for ArcXml. ArcXML is designed as the protocol for data exchange between the ArcIMS Spatial Server, Application Server, and Application Server connectors.

Basic Guide to ArcXML
Learning how to work with ArcXML is integral to customizing ArcIMS, ESRI’s web mapping software. Introduce yourself to this flavor of XML here.

GeoSciML
GeoSciML is a markup language specific to the geosciences and is designed to support the exchange of geoscience information.

GML
GML or Geography Markup Language is an XML based encoding standard for geographic information developed by the OpenGIS Consortium (OGC). The objective is to allow internet browsers the ability to view web based mapping without additional components or viewers.