20.7. What's the meta? Nothing. What did you think was the meta?[3] [3] Sorry about that! There are two "meta" words that come up regularly when computer types talk about XML: metadata and metalanguage. 20.7.1 Metadata Metadata is data about data. The date, publisher's name, and author's name of a book are metadata about the book, while the data of the book is its content. The DTD and markup tags of an XML document are also metadata. If you choose to represent the author's name as an element, then it is both data and metadata. If you get the idea that the line between data and metadata is a fluid one, you are right. And as long as your document representation and system let you access and process metadata as though it were data, it doesn't much matter where you draw that line. Be careful when talking to database experts, though. In their discipline "metadata" typically refers only to the schema. 20.7.2 Metalanguage You may hear some DTDs or schemas referred to as languages, rather than document types. HTML is a prominent example. There's nothing special about them, it is just another way of looking at the way a markup language works. Remember that an XML document is a character string that represents some conceptual document. The rules for creating a valid string are like the rules of a language: There is a vocabulary of element type and attribute names, and a grammar that determines where the names can be used. These language rules come from the DTD or schema, which in turn follows the rules of XML. A language, such as XML, which you can use to define other languages (such as DTDs), is called a metalanguage. XML document types are sometimes called XML-based languages. | 
