Correct Use Of Meta Description
Meta Description: The Power to Convert
Meta Descriptions are overlooked as an effective means to get more site traffic. Because search engines no longer place much value on meta descriptions for search results and placements, meta description markup is often ignored, or left to the CMS to generate automatically.
The simple fact is meta description markup is commonly the first impression a person using a search engine gets of your website.
Search engines (including Google) regularly use the content of the meta description field to provide the searcher with a snippet of information about the page returned in search results. The first thing a visitor sees that can give a good reason to choose your website over the rest of those listed in the search engine results pages!
What is Required for Effective Meta Description
If Meta Descriptions are to be effective in converting searchers to visitors, the description must tel the searcher in a few words what they will find on the linked page. Meta descriptions that are generic to the website are not what are required. The text must concisely SUMMARISE the CONTENT of the web or blog page.
I often see meta description that are repeats of old style meta keywords, try to provide a total overview of the website (That meta description goes on the HOME PAGE, and nowhere else!). Worse still, these generic keyword groups are repeated everywhere.
Meta Descriptions must be unique
Every webpage must have a UNIQUE Meta Description, the same way every page must have unique content. Duplication of content is not a good idea - Google will penalise the pages, even the entire website, if too much 
duplicated content exists and is not correctly controlled using other meta markup. Why do you think it is OK to duplicate meta descriptions everywhere - search engines still see meta descriptions, even if they don't use it for SERP. Duplicate meta descriptions can be considered spamming, just like keyword duplication spamming led to this meta data becoming redundant for search engine listing.
Let's say you have two or more pages listed in search engine results. Each has very similar or identical snippets displayed. One of these was found by Google for the main topic of the search tern, the other(s) found because it contained some words relevant to the term, but is not specifically about the topic.
How does a visitor choose which is the page about the subject being searched for. They only have the (nearly identical) snippets to attract them. The visitor chooses the wrong page because the meta description led him/her to expect to find the information required. Landing on the linked page they discover it bears little if any relevance to the wanted content. Does this person stay on your site or go back to Google for another search. When they find your pages listed again, will they try one of the other links to your site, or do they now have an expectation of disappointment?
Short, Precise and Informative
Short, Accurate and Informative is the goal to aim for when writing a meta description. You only have a limited 
number of characters to use. The common suggestion by SEO specialists, carried through to SEO modules and plugins is no more than 140 characters including spaces, punctuation etc.
This limit is not strictly accurate; Google at least does use longer snippets for the description field in search results, closer to the older limit of 240 characters. The 140 character limit should be a rule of thumb - try to put the main content description in the first 140, and if more are absolutely essential, make sure they are after this number.
Meta Descriptions should include the main key phrase the article or page is about. It adds to key-phrase density, improving the chance of the page ranking well in SERP.
There is absolutely no point in using a description that does not support the main key-phrase (The TOPIC of the page or article). If it mentions something the page is not SPECIFICALLY about, it is useless - in fact worse than useless as you will dilute key-phrase cluster density with irrelevant terms. Here is an example of wasted description
The page is an article about the impact the economy has had on regional computer sales posted on a computer store's website - The meta description reads:
Starting off wasn't too bad; the main topic got a mention, then the balance of the description went into total irrelevancy. Yes, it may be important to the store owner to tell people that is what the site is about, and they can find those things. But for the purpose of the article, it is useless. The page will not get found for those terms, and telling people looking for information about the economic impact on the computer industry that the page is a place to look for products is NOT going to attract a visitor.
The store owner needs to create a SINGLE page specific to looking for each of the product categories mentioned, and use a strong motivational description to get searchers to click through to the page from search engine results.
+Mike Otgaar
