It has found great use in the world of B2B EDI, or in English, Business to Business Electronic Data Interchange. Although not it's original intent, XML is linking incompatible databases together in order to streamline many processes between vendors and suppliers. XML is only going to grow in its importance. XML is only part of the picture. XLinks, XPointers, DTD, and Schema to name a few.
Good instruction is invaluable, and difficult to find.
I hear XML is still in development. The majority of XML is finished. Besides, the sooner you get started with XML the better. The standards are quickly being finalzied by the W3C so your best bet is to get in on the ground floor. So glad you asked.