A page's description meta tag gives Google and other search engines a summary of what the page is about. Whereas a page's title may be a few words or a phrase, a page's description meta tag might be a sentence or two or a short paragraph. Google Webmaster Tools provides a handy content analysis section that'll tell you about any description meta tags that are either too short, long, or duplicated too many times (the same information is also shown for title tags). Like the title tag, the description meta tag is placed within the head tag of your HTML document.
The beginning of the description meta tag for our homepage, which gives a brief overview of the site's offerings
Description meta tags are important because Google might use them as snippets for your pages. Note that we say "might" because Google may choose to use a relevant section of your page's visible text if it does a good job of matching up with a user's query. Alternatively, Google might use your site's description in the Open Directory Project if your site is listed there (learn how to prevent search engines from displaying ODP data). Adding description meta tags to each of your pages is always a good practice in case Google cannot find a good selection of text to use in the snippet. The Webmaster Central Blog has an informative post on improving snippets with better description meta tags.
Snippets appear under a page's title and above a page's URL in a search result.
A user performs the query [tutorial trick seo]
>Our homepage appears as a result, with part of its description meta tag used as the snippet
Words in the snippet are bolded when they appear in the user's query. This gives the user clues about whether the content on the page matches with what he or she is looking for. Below is another example, this time showing a snippet from a description meta tag on a deeper page (which ideally has its own unique description meta tag) containing an article.
Good practices for description meta tags
Accurately summarize the page's content - Write a description that would both inform and interest users if they saw your description meta tag as a snippet in a search result.
• writing a description meta tag that has no relation to the content on the page
• using generic descriptions like "This is a webpage" or "Page about baseball cards"
• filling the description with only keywords
• copy and pasting the entire content of the document into the description meta tag
Use unique descriptions for each page - Having a different description meta tag for each page helps both users and Google, especially in searches where users may bring up multiple pages on your domain (e.g. searches using the site: operator). If your site has thousands or even millions of pages, hand-crafting description meta tags probably isn't feasible. In this case, you could automatically generate description meta tags based on each page's content.
• using a single description meta tag across all of your site's pages or a large group of pages