If you've looked at HTML documents, you're familiar with the basic concepts of using tags to mark up the text of a document. This section discusses the differences between HTML documents and XML documents. It goes over the basic rules of XML documents, and discusses the terminology used to describe them.

One important point about XML documents: The XML specification requires a parser to reject any XML document that doesn't follow the basic rules. Most HTML parsers will accept sloppy markup, making a guess as to what the writer of the document intended. To avoid the loosely structured mess found in the average HTML document, the creators of XML decided to enforce document structure from the beginning.

(By the way, if you're not familiar with the term, a parser is a piece of code that attempts to read a document and interpret its contents.)