Why does Google care about meta descriptions?

Google wants snippets to accurately represent the web results.They frequently prefer to display meta descriptions of pages (when available) because it gives users a clear idea of the URL's content. This directs them to good results faster and reduces the click-and-backtrack behavior that frustrates visitors and inflates web traffic metrics. Keep in mind that meta descriptions comprised of long strings of keywords don't achieve this goal and are less likely to be displayed in place of a regular, non-meta description, snippet. It's worth noting that while accurate meta descriptions can improve clickthrough, they won't affect your ranking within search results.

Google can read both HTML and XHTML-style meta tags, regardless of the code used on the page

Google Do

  • Differentiate the descriptions for different pages.

  • Create descriptions that accurately describe each specific page. Use site-level descriptions on the main home page or other aggregation pages, and consider using page-level descriptions everywhere else.

  • Prioritize. At the very least, create a description for the critical URLs like your homepage and popular pages.

  • Include clearly tagged facts in the description. It's a great place to include structured data about the page. For example, news or blog postings can list the author, date of publication, or byline information. This can give potential visitors very relevant information that might not be displayed in search results snippets otherwise.

  • Make it human-readable and make it look nice in search results. Supplement the title.  Start with a capital and use proper spacing and punctuation. After a comma comes a space for example.

Google Don't

  • Duplicate the text already in the page title in the Meta Description. It would not look good in search results so it will probably not be used by Google.

  • Do not repeat information within the description itself.

  • Do not use generic descriptions like "This is a web page" or "Page about webmaster tools".

  • Do not use a single description meta tag across all pages or a large group of pages

Matt Cutts on Meta Descriptions