I do not know the theory of XML

There are 10 popular XML Interview questions has to be aware of, especially I am JAVA programmer. Remember last time after my graduated I was attending an interview for a Web programmer position, I do not know how to answer this question: What is XML, What is DTD, and What is DOM although I know how to write the code. But interestingly I'm not able to answer this question and this seems that I don't even know the theory of XML.