use they are very important.that is what google shows in its results pages for your site description.If this is left empty,then it will pick a description from your website.So its best to give a description you have constructed yourself.
It's actually very rare that they use the meta description. Usually in SERPS they give the snippet of text in the content of your page that matches the search phrase.
that is only if you have no meta description tag.Perform any search on google now and you will see the same results when you look at the meta tag.
No, if the search phrase is in the description, then it might show the meta description, but the vast majority of search results show a snippet of text content that contains the search phrase, regardless of whether or not the page has a meta description.
Very few search engines use meta description as a ranking factor in their ranking algorithms. Google does not. Like the meta keywords, since it is easily spammed and not shown anywhere on the web page, it is ignored by most from a ranking perspective. However, meta descriptions are VERY important... not for ranking... but for click-thru-rates. Optimizing meta descriptions for each page on your site can lead to increased click-thru-rates. You want to optimize them for each specific page. Don't use the same one for every page. Every keyword if possible from the title element of a page whose meta description is being optimized should be included in the meta description. If the title element is optimized properly along with the rest of the page, having all of the keywords from the title in the meta description will increase the number of times it is shown as the Google snippet in the search results. And the meta description should include a call to action so that when it IS shown as the snippet at Google, people will be more inclined to click-thru to your site.
Yes, you should add meta description in all other pages in your website. Treat each page separately. Although, it is not take into account in ranking, it will give clearer picture for your visitors of that particular page. I suggest to keep the meta description at 160-350 letters length.
I agree with all the above however treat each page as a unique page and you will see your rankings increase over time.
Meta description is not useful for ranking...however it is the google snippets in SERPs...so you ca optimize it so that user click on the link..and make sure you are using different meta description for each page..