A moment ago, I’ve read somewhere in forum says that meta tags are not important as now. These things bother me a lot. My friend and I are having a debate about the value of the meta description tag. He says that search engines don't index them anymore and so you don't need to use one. I think that he's wrong and that search engines will use them in the search results pages to describe your site, but if you don't include one, you might find a snippet of text from your page being used instead.
But Kalena's answer that it depends on each search engine. For example, Google and Yahoo will sometimes use part of your META Description tag, but they will usually use a snippet of text on your web page that includes the exact search query entered. But if your site is listed in the Yahoo Directory, Yahoo will often use your editor-created description from there. If your site is listed in the ODP, Google may use your ODP description.
As Ammon Johns says in this post at forums, the most valuable use of the META Description tag is to write it in a way that is going to convince relevant visitors to click on your link if it is shown in the SERPs. Always write a unique META Description for each page on your site and make sure it accurately describes your page and the content users are going to find on it so you only attract clicks from qualified visitors looking for exactly the information you are offering.
Ron says: There's some excellent 'meat' in those links you give, he particularly like the 155 character suggestion in the last one.
As you suggest, I try to limit the meta description to 155 characters, which is what Google will likely show if it includes the keyword phrase being queried. Since most of the other snippets on the SERP will likely be gibberish, your well-written 155 characters will likely get the click.
It’s better to have a unique meta description tag for each of page, but it doesn't mean that search engines rank web pages based on it. Search engines looks the page content as a whole, and also at those links pointing to that particular page.
The Description meta tag can be used on all the major search engines to increase click-through rates, and more importantly, to pre-qualify visitors who won't be interested and thus lower the bounce rates and increase conversion rates.
However, all the major search engines also have certain quirks about when to use more trustworthy data for snippets too. Yahoo will use the description from its Yahoo directory for example. Google will use the description from a dmoz listing for the precise URL (thus usually only a root domain), and so forth.
You have to know enough to know when to use the NOODP and other snippet controlling meta tags; to understand how Google determine the best snippet to use on a searc term by search term basis; and to recognise that no matter what, there are times when the snippet is going to be pulled from the page.
There are still mllions upon millions of web pages out there that use the same meta content for every page, describing the site rather than the page, and even then making a poor job of it. There are millions more pages out there that are auto-generated by poor CMS systems and may have no meta content at all.
The search engines have to deal with all of these pages and issues, and have developped a system that is about te best all-around system they could create within realistic resources.
Making the systems and limitations of the search engines work to your benefit is what SEO is all about.