Description, Keywords

Meta Description: Search Engines (especially Google), tends to ignore them at this time, but they are still making use of them for snippet compilation, also called “call to action“.
When displaying result, search engines tend to make use of this tag to describe the website profile, and is shown below the “Page Title” (also the link to your website) as a snippet.
If the meta description is not found on your page search engines will decides to show a snippet from the page content itself (a snippet of what is considered to be more relevant to the search query).
The downside of this is that the snippet might not have that “call to action” that you are going for.
Another use of this tag, is, when submitting your website to directories, as some of them will pick-up and use the meta description you have on that page as their directory listing description.
As the page title, meta description should be also unique across the website for each page. It’s like a short description of the product or service itself that all-in-all gives the user the exact idea what the page it’s all about.
The length of this tag should contain approx. 20-25 words and should not exceed more than 150-160 characters (including white spaces).