The meta description tag contains a keyword rich summary of your web page which is useful as an aid to encouraging user clicks as well as SEO optimisation and search engine spidering. The meta description is written in the head area of the web page, commonly just below the title tag, and is therefore not displayed in the browser window. The contents of the description can often be seen in the search engine results pages however, and so a well written description can be the difference between a click on your site and a click on a competitor in the same search results. According to Google's webmaster guidelines, Google's preference is to show a 'snippet' of the meta description in the search results as an accurate idea of the page's content.
The description is written as the quote enclosed value of the description meta tag's content attribute in the head area of a web page as follows:
<meta name="description" content="tell the world what your web page is about" />
The meta description tag must be used on every page throughout this site, as it remains a useful addition to the unique page content itself. The meta description should not repeat the page title, but rather should compliment and expand the page title and describe the theme of the page. Always ensure the most important ideas you are trying to relate are found at the beginning of the description, and that the description is an accurate portrayal of the content on the page. Care should be taken to give every page on your site a unique and accurate meta description tag, as duplicate descriptions are always found and frowned upon by Google and the like. Once your site has been fully indexed by Google, you can check for instances of duplicate meta tags inside the Google webmaster tools content analysis. You do not necessarily have to write complete sentences, as for example, if the description is of individual product then a summary of the product specifications might be the most appropriate meta description but may not be exceptional prose. If you are wondering how long a meta description should be, or how many words it should contain, there are no definite rules. Although the meta description is not one of the main SEO ranking criteria, you should still concentrate on the content of the meta description, writing one or two sentences that describe the page, that will encourage the viewer to click on your site and not a competitors. A well written and unique meta description on every page of your site will encourage the most clicks. If your meta description is too long, too short, or duplicated on another page in your site, this will be highlighted for you in the Google Webmaster Tools Content Analysis.