The META element is possibly the least understood and most abused component of the Hypertext Markup Language. Although META has been part of HTML for nearly a decade, many users (and would-be teachers) don't appreciate the nuances of META tagging, either neglecting it all together, or weighting down their web pages with useless extensions of the tag.
Most references on the META element will claim that META exists to "describe your page to search engines", or some similar oversimplification. The true purpose of META tagging is both more subtle and more expansive than many people realize.
The META element exists to provide web authors with an extensible container for identifying metainformation about an HTML file. Metainformation is, as the cliché goes, "information about information". Whereas the BODY of a web document contains the information about the subject of the document, its META tags contain information about the document itself.
Metadata can be used by any entity that has access to it, not just search engines, so various applications of the META tag may be meant for search engines, browsers, web servers, authoring systems, or even human readers. To provide the flexiblity necessary for such an expansive mission, the META element itself is designed to be structured and extensible -- anybody who needs to add a new kind of metadata to their documents just has to create a name for it, and use the META element to associate the names with values.
The syntax of the META element is very simple. Most of the time, you just need a tag like this:
<meta name="label" content="data">
Where label identifies the type of metainformation you're communicating, and data is the actual metainformation.
In some cases, name= is replaced by http-equiv=. Technical details of META tag syntax are available on a separate page.
As of 2002, there are dozens, possibly hundreds, of different labels (names) being used to identify metadata on the Web. Creating a catalog of known labels is an ongoing project here at Websnob, but here's what we have so far, and how we're categorizing them:
The most popular META tags are META tags that try to control search engines. There are only three or four such tags that have widespread acceptance, but many search engines have proprietary tags. Several groups have also proposed advanced metadata schemas for indexing documents.
The second most common use of META tags is to use META tags to try to control web browsers. Most of these tags are used to control browser caching or page presentation.
Somewhere in the fuzzy universe of the
wireless web, there are
a bunch of META tags for the mobile web.
Finally, some META tags are useless. The useless tags are mostly the product of web authoring software that fills web pages with useless advertising tags, or encourages authors to make themselves feel important by adding tags nobody ever sees. A few tags are simply legends, passed around by authors following poorly-researched design advice.
There is, of course, one big list of known META tags you can consult to find out the purpose of mystery tags you encounter on the web.