XML in a Nutshell, 3rd Edition

The Benefits of XML

XML is a metamarkup language for text documents. Data are included in XML documents as strings of text. The data are surrounded by text markup that describes the data. XML’s basic unit of data and markup is called an element . The XML specification defines the exact syntax this markup must follow: how elements are delimited by tags, what a tag looks like, what names are acceptable for elements, where attributes are placed, and so forth. Superficially, the markup in an XML document looks a lot like the markup in an HTML document, but there are some crucial differences.

Most importantly, XML is a metamarkup language . That means it doesn’t have a fixed set of tags and elements that are supposed to work for everybody in all areas of interest for all time. Any attempt to create a finite set of such tags is doomed to failure. Instead, XML allows developers and writers to invent the elements they need as they need them. Chemists can use elements that describe molecules, atoms, bonds, reactions, and other items encountered in chemistry. Real estate agents can use elements that describe apartments, rents, commissions, locations, and other items needed for real estate. Musicians can use elements that describe quarter notes, half notes, G-clefs, lyrics, and other objects common in music. The X in XML stands for Extensible. Extensible means that the language can be extended and adapted to meet many different needs.

Although XML is quite flexible in the elements it allows, ...