This chapter shows you how to write simple XML documents. You'll see that an XML document is built from text content marked up with text tags such as <SKU> , <Record_ID> , and <author> that look superficially like HTML tags. However, in HTML you're limited to about a hundred predefined tags that describe web page formatting. In XML, you can create as many tags as you need. Furthermore, these tags will mostly describe the type of content they contain rather than formatting or layout information. In XML you don't say that something is italicized or indented or bold, you say that it's a book or a biography or a calendar.
Although XML is looser than HTML in regard to which tags it allows, it is much stricter about where those tags are placed and how they're written. In particular, all XML documents must be well- formed . Well- formedness rules specify constraints such as "Every start-tag must have a matching end-tag," and "Attribute values must be quoted." These rules are unbreakable , which makes parsing XML documents easier and writing them a little harder, but they still allow an almost unlimited flexibility of expression.