XML

XML (an acronym for "eXtensible Markup Language") defines a set of standard rules for creating markup languages. A markup language is a mechanism of labeling content, typically text, and primarily so that it can be understood and processed by software. One common markup language you've probably seen before is HTML (HyperText Markup Language), which looks like this:

<p>I <b>really</b> dislike <a href="https://toc.oreilly.com/resources/drm.html">DRM</a>.</p>

Which when viewed as part of a Web page, looks like this:

I really dislike DRM.

Those words inside of the brackets are called "elements".

There are a wide variety of types of content to describe with a markup language, but there's a lot of benefit to using a common type of markup language, regardless of the content type. For example, in the case of a document meant for the Web, to describe that content you'd want to describe things like "paragraph" (<p>), "heading" (<h1>), and "ordered list" (<ol>).

But in the case of corporate financial data, you'd want to describe things like "inventory" (<ifrs-gp:Inventories>) and "total current assets" (<ifrs-gp:AssetsCurrentTotal>). By using XML to define the markup, even though the content is quite different, a common set of tools and techniques can be used for creating and processing both types of content.

Although few people work directly with XML regularly, most everyone using the Web actually uses XML every day. For example, the RSS feeds used to track blog updates are XML documents. Here's a snippet of the TOC feed:

<feed xmlns="https://www.w3.org/2005/Atom">
<title>Tools of Change for Publishing</title>
<link rel="alternate" type="text/html" href="https://toc.oreilly.com/" />
<link rel="self" type="application/atom+xml" href="https://toc.oreilly.com/atom.xml" />
<id>tag:toc.oreilly.com,2008-01-24://40</id>
<updated>2008-07-18T21:01:03Z</updated>
<subtitle>Tools of Change for Publishing from O&apos;Reilly Media: Technology is transforming publishing. 
Are you ready for the future? </subtitle>
<generator uri="https://www.sixapart.com/movabletype/">Movable Type 4.1</generator> 
<entry>
<title>[TOC Directory] Recent Additions</title>
<link rel="alternate" type="text/html" href="https://toc.oreilly.com/2008/07/
toc-directory-recent-additions-2.html" />
<id>tag:toc.oreilly.com,2008://40.25177</id> 
<published>2008-07-22T14:30:00Z</published>
<updated>2008-07-18T21:01:03Z</updated>

XML is often used as a file storage format (either as the primary format or an alternative) in word processing and desktop publishing software like Word, InDesign, OpenOffice and even Excel. All use the rules of XML to define their file formats, though each uses their own particular names for their elements, and have different rules about how their elements can appear within a document. Indeed, one of the big advantages of XML is the ability to use a standard set of tools for defining the rules of a particular document ("tables are not allowed inside of sidebars" or "every image must have a caption"). Before XML, such rules were often codified in style manuals, or perhaps enforced with custom software such as Word macros or InDesign scripts.

For more technical information, see this in-depth technical overview of XML, this "Learning XML" course from the O'Reilly School of Technology, or "Learning XML, 2nd Edition".

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