XML

Tags

XML is a way of describing how to describe things. You can think of XML as the alphabet which you can use to create words with. Then you take these words and actually perform the actual act of describing things. These uses of XML are often referred to as XML applications. One of these XML applications is XHTML, which is a markup language made for use on the WWW. E.g. if I want the word "blind" to be bold in the sentence "Three blind mice." I would write

Three <b>blind</b> mice.

The <b> and </b> are called tags, a start tag and an end tag. The starttag, <b>, turns bold on and the endtag, </b>, turns off the bold property. The XML rules regarding tags are fairly simple, you must turn off, or close, all tags that you have opened and you must do it in the reverse order of how you opened them. An example with both bold and italic in XHTML:

<b>Three <i>blind</i></b> <i>mice</i>.

An empty tag <x></x> may be compressed into <x/>. This is useful for tags that doesn't need any content to be meaningful, e.g. the line break tag <br/> in XHTML.

Attributes

Tags can be made more specific by the use of attributes, which consists of an attribute name and an attribute value, or argument.

<b lang="en-uk">Three blind mice.</b>

Note that either two " or two ' may be used around the argument, depending on the contents. Example:

Did you hear that someone actually donated a pie to
<person name='William "Bill" Gates'>him</person>?

Entities

In addition to tags XML also specifies entities, which are used as a constant that represents something else. It is for example forbidden to use the characters < and > for anything else than making tags in XML. Thus there must be some other way to write these characters when you need them, &lt; (less than) and &gt; (greater than).

More information about both XML and XHTML is available at www.w3.org.