XML -- eXtensible markup Language -- is an exciting development in web technology. It is the youngest and most comprehensive of markup languages. (Markup refers to anything on a document that adds special meaning to a particular piece of text; for example, bold text is a form of markup). XML got its name Extensible Markup Language from the characteristic that it is not restricted to a fixed set of tags, as is HTML (Hypertext Markup Language). An XML user creates his own tags according to need. A tag is a sequence of characters in a markup language used to provide information, such as formatting specifications, about a document.
Markup languages are roughly classified into three types:
Stylistic – defines character presentation; for example bold, italics, underline, font etc.
Structural – defines the structure of the document, such as heading and paragraphs.
Semantic – informs us about the content of the data, like specifying a title.
XML 
XML Strengths and Weaknesses with DOM, ASP and XSL
