Entering search engine meta-data
From Joomla! Documentation
Metadata is information about the website in the header portion of the web page. This information is used by search engines to determine the relevance of the page to the search request it is processing. This data is collected by various means by search engines such as Google, Yahoo and Bing. The following is a partial list of things to consider when building a website.
Well-constructed title tags contain the main keyword for the page, followed by a brief description of the page content. It will be less than 65 characters and avoid using stop words such as: a, if, the, then, and, an, to. Your title tag should also be limited to the use of alphanumeric characters, hyphens, and commas.
Good description tags contain information about the page's content and persuade search engine users to visit your web site. They should be between 25 and 35 words in length.
Each page of your site should use at least the H1 heading tag for the search engines that examine it when crawling your site.
Pages should have between 300 and 700 words of descriptive content that contains relevant keywords.
Each page of your site should contain links to all other pages so search engine spiders can find every page. This is a critical step for the proper indexing and page rank distribution of your site.
It's important to use two site maps for your website—an XML version and a static version. The XML table protocol is explained in detail on sitemaps.org. The static version should sit on a static HTML page and contain links to all pages of your site.
It's important that search engine spiders find your Robots.txt file that guides spiders to pages and directories you want crawled and denies them entry to areas that are off limits.
Because search engines treat websites as a grouping of pages and not a single entity, each page on your site should be unique so that the tags and content differ between each page. Doing so increases the number of pages that will rank.
Pages should contain 300 to 700 words of unique and descriptive content.