"Learning XML" takes maybe a day. The very basics can be understood in a half hour. The trick is that XML isn't so much of a language as a framework of rules behind a whole family of languages used to describe and manipulate data using markup tags (hence eXtensible Markup Language).
The most common languages used to manipulate XML files are XSLT (which converts an XML document into a different kind of XML document, or into another markup language like HTML, or into text) and XSL-FO (for defining print and PDF layouts). If you're a Firefox user you may have heard of XUL, a language used to create user interfaces. In order to use these, you'll need to understand XPath, and the day is coming when XLink and XQuery will be useful.
XML used to describe data is defined in what is called a "schema," another set of rules which governs what the names of the data elements are, what attributes they have, and how they can be used, and at the present usually defined in DTDs. You've already heard of one of these XML formats: XHTML. XHTML is very similar to HTML, but because it is a kind of XML it follows XML's stricter rules, for instance your "tags" must be all lowercase and all tags must be closed.
Other common kinds of XML for describing data include RSS, RDF, SVG, MathML.