The eXtensible Markup Language (XML) is a meta-markup language defined by the World Wide Web Consortium (W3C). It is not strictly a markup language itself, but rather a set of rules for creating markup languages. For our purposes a markup language is any language (HTML, for example) that uses tags surrounding text to convey information such as content or format. CNXML, the markup language used by the Connexions Project is an example of a language written in XML. There are many other examples at the W3C site. Here is an example of some markup in CNXML.

      <para>
	This is a paragraph in <term>CNXML</term>.  Notice that the
	markup contains tags that express the meaning of the text.
      </para>
      

<para> and </para> are the tags that the enclose the text. In XML, tags are always marked by angle brackets (also known as < and >). Tags generally come in pairs: an opening tag will look like <tagname>. A closing tag will look like </tagname>, with a / preceding the tag name.

XML allows the separation of presentation from content. HTML has tags such as <u> and <i>, which underline and italicize text respectively. There is no content information expressed, only formatting. XML allows you to define your own language of tags to represent content. You could create a tag called <book> to represent book titles, and create a stylesheet (a separate formatting document), that says that every <book> tag should be italicized or underlined. Then when you want to change the presentation of that type of content, you just change one small part of the stylesheet. Also, if you make tags that convey the content of the document, you can enable better searching. For example, you might look for the md:author of a document by looking at the md:author tag.

XML has a few rules that apply to all of its languages, including CNXML. If a document satisfies these rules, then it is well-formed. XML documents are required to be well-formed.

It is possible to define a set of rules that apply to all of the tags in a particular XML language. These rules can be defined in a couple of different ways. The most common way is to use a DTD (Document Type Definition). Any document which follows all of the rules for that language is called valid. A document is not required to be valid in order to be XML. However, it is generally a good idea.

An entity is a way to escape characters. Entities always have the same basic form. An entity will start with an ampersand and end with a semicolon. For example, &amp; will escape the & character. So if you type &amp; in the source file, you will see & when you view it in a browser. There are several entities that are predefined and will work in any XML file. They are &amp; (&), &lt; (<), &gt; (>), &quot; ("), and &apos; ('). These are the same entities that are defined for HTML. In addition to these basic symbols, you can define your own entities using a DTD. These self defined entities can point to other symbols, or they can be short cuts to a larger piece of text.

You can also include unicode characters in XML. Unicode is a project to define a unique code for every character in any human language. Unicode is very useful any time that you need to use a symbol that is not a part of ASCII.

Unicode entities have an extra # sign. So they look like: &#134;. Or if the code is in hexadecimal, there is also an extra x causing them to look like: &#x03C0;.

The World Wide Web Consortium(W3C) is responsible for XML.

See www.unicode.org for more information on Unicode.