Whump thinks XML Catalogs matter…
If you’re wondering what XML Catalogs are (the Oasis spec is highly readable), I would describe them as a means for customizing the resolution of external resources referenced in XML documents.
For example, imagine an XML instance document uses a Doctype to specify a DTD to which the instance doc conforms (like, say, XHTML 1.0 Strict). One might customize one’s XML parsing software with an XML Catalog that would tell the parser to use a local version of the DTD rather than fetching the remote DTD referenced in the Doctype. This allows offline parsing, and is generally much faster.
This is particularly relevant with the recent Netscape RSS 0.91 DTD debacle.
XML Catalogs can help resolve more than just DTDs, though… the spec is clearly designed to allow custom resolution of any external resource like inline-images or graphics and more.
Oh, and by the way… if you use TextMate, my plugin TeXMLMate includes XML Catalog support. 