Meta Description Tag
Although this tag does not weigh heavily in search engine ranking algorithms, it remains a powerful element of an effective SEO campaign. As the title tag, Meta description tag will always appear in search results. In general what you see in the search engine ranking pages are the Title tags then the description tag followed by its specified link page. It becomes a very important element to help attract visitors to click the link on your site.
Those who take the time to look through search results, read the titles and descriptions to find sites that are most likely to give them the information they seek are more likely that to be a targeted visitors.
The general rule is that you want every description of your tags to be unique. The description must be maximum 150 characters summary of what visitors can expect to find on this page and this page only. Descriptions for each page must be unique from each other. Be sure to summarize the page in a unique way.
But is not always advisable to put description tag on every page. There are some cases where it would be a better not put description tag. If you are targeting long tail keywords, then do not put meta tags. Description tags would be best use when targeting broader keywords.
An explanation to this is that if you go after long tail phrases in your article or blog post, there is a lot of variations that can come up in a description of 150 characters. If there are no description tags created, search engines will imports from the page snippets based on search made if those long-tail words are on the content.