By pelegri on Sep 11, 2007
Quoting from the Oasis Standard, an XML Catalog...
maps external identifiers and URI references to (other) URI references. It also defines a format for catalog entry files in terms of [XML] and [XML Namespaces].
A catalog is particularly useful for mapping well-know URIs for Schemas; a typical case are those for the Java EE Deployment Descriptors. This is very useful for performance reasons and required in disconnected mode.
Norm was the editor for the Oasis standard and also provided a popular implementation that is included in GlassFish v2. Check out his latest writeup where he provides some background on the XML catalog and shows How to Use it.