In working with our clients to create highly visible websites, we've found that there's a lot of confusion out there about meta tags and their value in search optimization.
Meta means "about"; thus, meta tags are html tags that give information about your webpage to search engines. The three most common meta tags are titles, descriptions, and keywords.
A page's title is one of the most important factors for search engine optimization, and is weighted heavily by Google to determine a site's relevance and content. Meta titles not only affect ranking, but are displayed as the headline (the part you click on) in search results. Use unique, keyword rich titles for each page; if you want to include your site name, place it to the right of the title that describes the page's specific content.
Meta descriptions don't have much effect on a site's rank, but Google often uses them to characterize a page in search results, and an appealing description can garner you more click-throughs. When writing a meta description, think about what will make someone want to click on your link, rather than the twenty others on the page. Make sure it's quick and to the point—it shouldn't be more than two sentences. For more details, check out Google's guide to picking good meta descriptions.
Many people think that meta keywords are the secrets to search engine success, and historically, they did determine rankings. Unfortunately, these keywords became too easily abused—people simply stuffed in popular but irrelevant keywords like "Britney Spears" to get traffic—and Google now openly states that meta tag keywords are no longer a factor in determining web rankings. So don't waste your time coming up with the perfect meta keywords—it's much more important to have well crafted meta titles and descriptions when it comes to seeing search results.