XML stands for eXtensible Markup Language, and is a rapidly maturing data format for structured document interchange on the Web. However, XML is in no way limited to Internet use. In fact, XML's main strength - organizing information - makes it perfect for exchanging data between different systems, regardless of whether the Internet is part of the picture.
I can't believe I once bought an actual book to learn XML. Well, at least I didn't buy a book to learn SGML, even though I had used SGML in the 90's. I worked at a company called Ligature that worked with digital dictionaries, some of which came in SGML format.