XML is one of the most important technologies to learn about. Like relational databases in 80s and 90s, XML is rapidly becoming the glue that binds web applications together. I just came across this link to a series of articles on XML Mastery by Frank Coyle.
Frank Coyle leads you to a complete understanding of exactly what you need to know to build and implement your own sophisticated XML solutions, whether for your personal web site or an enterprise-level organization. This introduction to the series briefly describes the relationships among the various technologies you’ll learn as you travel the seven steps to XML mastery.