Meta description - definition
Meta descriptions are commonly used by the search engines on the search engine results pages (SERPs) to describe the content of the page to the user.
Optimal length is 150 characters. Most search engines will cut off descriptions at that point.
Meta descriptions are NOT used by the search engines in ranking the page for SEO, but can be a significant factor in influencing a human to click on one result over the other.
Keyword usage is relevant, despite the lack of SEO influence and is important to conversion. Most search engines will bold user search keywords in your meta-description.