Definition: Bounce rate
It is the percentage of visits to a site that leave without visiting a second page. It must be as low as possible because it measures the interest for the site content. However it is debatable whether it is really a quality criteria because visits from search engines come to a site to find the exact response to a question so they do not need to go to another page if they have found it.
Definition: Url
Uniform Resource Locator, a site address that contains the name of the domain and the name of the file. For example: http://www.optimanova.com/services/
Definition: Snippet
Name given by Google to the text that appears below a web title in the search results. It may come from the meta description, from Dmoz or from the page content itself.