Following these guidelines will help Google find, index, and rank your site. Even if you choose not to implement any of these suggestions, we strongly encourage you to pay very close attention to the “Quality Guidelines,” which outline some of the illicit practices that may lead to a site being removed entirely from the Google index or otherwise penalized. If a site has been penalized, it may no longer show up in results on Google.com or on any of Google’s partner sites.

Link: Webmaster Guidelines