In August, Google released a new tool within Google Webmaster Tools that makes it possible for webmasters to see "manual actions" that have been taken by Google against their website and request that Google conduct a review of those actions. Manual actions are actions Google takes against spammy sites that violate Google's webmaster guidelines. Those actions often include a major downgrade in Google PageRank. Google does this when it finds a website is boosting its Google rank through methods like keyword stuffing, duplicated content and paid keyword links, among others.
If this happens to your site, you can apply to win back Google's trust and get out of Google jail. This infographic shows some tips on how to do it. Of course, the better route is to stay off Google's hit list in the first place, by creating high-quality, optimized content and growing PageRank the good old fashioned way: With time and hard work. (Find out more about how to do that in 3 SEO Tactics Google Loves.)