XML itself is a specification for developing languages. Per se, XML itself is not very useful.
However, several XML languages are highly useful indeed. For instance, XHTML is the XML version of HTML. SVG is an XML language for Scalable Vector Graphics (and very nice ones, too). XSLT transforms from one language to another.
Outside of presentation languages such as SVG, the largest application of XML is for creating languages for storing data, and conveying that data between applications (such as a server and a browser, in one very limited sense) in a standardized format.
Does this help?