Metadata are data about data, for instance a description of a website. Metadata are important for search engines and are written in XHTML. Exactly how important metadata are in the rating by search engines is debatable. However, metadata represent a good way to describe your website in short and precise words. If you check out the HTML source code of a Joomla! site, you will see the following meta tags in the top area.

<meta name="generator" content="Joomla! 1.5" />
<meta name="description" content="Joomla!
- the dynamic portal engine and content management system" />
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta name="robots" content="index, follow" />
<meta name="keywords" content="joomla, Joomla" />

Separate tags with the description and keywords attributes can be used.
Global Site Meta Description: This description of site content is often displayed by search engines. You should pay special attention to this tag, since the user who has entered a search term often uses this information to make a decision of whether to visit a site or not. You can add further information about every individual page to the global description.
Global Site Meta Keywords: Keywords are very important for a document. You should list the most important concepts of your website with them. Some search engines index keywords in particular. The individual words are separated by commas; several words can be entered between two commas with normal spaces in between. The keywords should be limited to a maximum of 1000 characters; more than that will not be read by the search engines. Note that the use of fewer keywords helps each individual word get a higher priority in the search engine. Deliberate carefully about which are the most often used keywords and which are likely to be searched for most. You can add more keywords to the global keywords for every individual page of your website.
Show Title Meta Tag: With individual content pages, the content title is blended in as a meta tag, for instance:

<meta name="title" content="Welcome to Joomla!" />

Show Author Meta Tag: With individual content pages, the author's name is blended in as a meta tag, for instance:

<meta name="author" content="Administrator" />