Descriptions should be defined for each page
A page's description meta tag gives Google and other search engines a summary of what the page is about. It consists of one or two sentences or even a short paragraph. In Google Webmaster Tools there is a useful content analysis section that tells you about any description meta tags that are either too short, long, or duplicated too many times (While this tool won't directly help you rank better, it might still help your site display better titles and snippets in search results, thus increasing visitor traffic). Like the <title> tag, the description meta tag must be placed within the <head> tag of your HTML document.
What are the merits of description meta tags?
Description meta tags are important because Google might use them as snippets(*) for your pages. That's uncertain because Google may choose to use some other text to display, such a more relevant section of your webpage or the site description in the Open Directory Project (if listed there). Adding description meta tags to each of your pages is a foolproof practice in case the search engine can't find a more relevant piece of text for the snippet.
Words in the snippet are bolded when they coincide with the user's query. This gives the user clues about whether the content on the page matches with what he or she was looking for, thus being an indicator of relevancy.
(*) A snippet is the text displayed under the bolded title of a web page within the search results pages of a search engine. An abstract of the web page, the description from the meta tag itself and/or parts of the page with the search keywords highlighted will be displayed.
The snippet quality could have a direct impact on the clickability of the search result being clicked. The tag should look like this:
<META NAME="Description" CONTENT="Your description goes here">
The search engine will probably prefer to display meta descriptions of pages when they are available because they give surfers a good idea of the webpage's content. This directs them to good results faster and reduces the click-and-go back behaviour that annoys the users. Beware of the fact that long meta descriptions made up mainly of strings of keywords don't achieve this goal and are less likely to be displayed in place of an excerpt (non-meta description) snippet.
On the other hand, while well-written meta descriptions can improve click-through rates, they won't affect your ranking within search results.
What are some good meta description strategies?
Differentiate the descriptions for different pages
Using identical or similar descriptions on every page of a site isn't very helpful when individual pages appear in the web results. In these cases we're less likely to display the boilerplate text. 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. You should obviously prioritize parts of your site if you don't have time to create a description for every single page; at the very least, create a description for the critical URLs like your homepage and popular pages.
Include clearly tagged facts in the description
The meta description doesn't just have to be in sentence format; it's also 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 the snippet otherwise. Similarly, product pages might have the key bits of information -- price, age, manufacturer -- scattered throughout a page, making it unlikely that a snippet will capture all of this information. Meta descriptions can bring all this data together.
The average person viewing a Google results page -- who might spend under a second scanning any given snippet -- is likely to skip an un-impressive result. What to do? Don't duplicate information, use enough information, and clearly tag and separate every bit of information.
Programmatically generate descriptions
For some sites, like news media sources, generating an accurate and unique description for each page is easy: since each article is hand-written, it takes minimal effort to also add a one-sentence description. For larger database-driven sites, like product aggregators, hand-written descriptions are more difficult. In the latter case, though, programmatic generation of the descriptions can be appropriate and is encouraged -- just make sure that your descriptions are not "spammy." Good descriptions are human-readable and unique.
Use quality descriptions
Finally, make sure your descriptions are... descriptive. It's easy to become lax on the quality of the meta descriptions, since they're not directly visible for the page's visitor. But since meta descriptions might be displayed in the snippet in the search results, a little more work on it can bring great results in showing a relevant snippet in SERPs, thus leading to better incoming traffic, both in quality and in quantity .