Google Penguin is a code name for a Google algorithm update that was first announced on April 24, 2012. The update is aimed at decreasing search engine rankings of websites that violate Google’s Webmaster Guidelines by using now declared black-hat SEO techniques involved in increasing artificially the ranking of a webpage by manipulating the number of links pointing to the page. Such tactics are commonly described as link schemes. According to Google's John Mueller, Google has announced all updates to the Penguin filter to the public. 
Penguin’s effect on Google search results
By Google’s estimates, Penguin affects approximately 3.1% of search queries in English, about 3% of queries in languages like German, Chinese, and Arabic, and an even bigger percentage of them in "highly spammed" languages. On May 25, 2012, Google unveiled another Penguin update, called Penguin 1.1. This update, according to Matt Cutts, was supposed to affect less than one-tenth of a percent of English searches. The guiding principle for the update was to penalize websites using manipulative techniques to achieve high rankings. The purpose per Google was to catch excessive spammers, but it seems some legitimate sites and SEOs have been caught with this latest algorithm change. Few websites lost search rankings on Google for specific keywords during the Panda and Penguin rollouts. It appears anchor text was to blame in these cases, as the links pointing to these sites concentrated on only one or a few keywords while the content of the websites was satisfactory. As the update focused on the quality of backlinks, so the result varied for different websites. Google specifically mentions that doorway pages, which are only built to attract search engine traffic, are against their webmaster guidelines. Regardless, many people still use this technique.
The difference between Penguin and previous updates
Before Penguin, Google released a series of algorithm updates called Panda with the first appearing in February 2011. Panda aimed at downranking websites that provided poor user experience. The algorithm follows the logic by which Google’s human quality raters determine a website’s quality.
The strategic goal that Panda, Penguin, and page layout update share is to display higher quality websites at the top of Google’s search results. However, sites that were downranked as the result of these updates have different sets of characteristics. The main target of Google Penguin is spamdexing (including link bombing).
Google’s Penguin feedback form
Two days after Penguin update was released Google prepared a feedback form, designed for two categories of users: those who want to report web spam that still ranks highly after the search algorithm change, and those who think that their site got unfairly hit by the update. Google also has a reconsideration form through Google Webmaster Tools for the 700,000 sites.
- The Penguin Update: Google's Webspam Algorithm Gets Official Name
- Webmaster Guidelines - Webmaster Tools Help
- Link schemes - Webmaster Tools Help
- Barry Schwartz (20 February 2013). "No, Google Hasn’t Released Unannounced Penguin Updates". Retrieved 29 April 2013.
- Another step to reward high-quality sites - Inside Search
- Google Penguin 2.0 - (Spanish)
- Search Engine Watch
- "Google Penguin Update 3 Released, Impacts 0.3% Of English-Language Queries". Searchengineland.com. 2012-10-05. Retrieved 2013-06-16.
- "Google's Matt Cutts: Next Generation Of The Penguin Update "Few Weeks" Away". Searchengineland.com. 2013-05-10. Retrieved 2013-06-16.
- Infographic: The Google Panda Update, One Year Later
- How Google Uses Human Raters in Organic Search - SEW
- Official Google Webmaster Central Blog: Page layout algorithm improvement
- Browser Size - Google Labs
- "Feedback on our recent algorithm update ("Penguin")". Docs.google.com. 2012-04-24. Retrieved 2013-06-16.
- Penguin Update Peck Your Site By Mistake? Google's Got A Form For That
- Google Penguin Update: Impact of Anchor Text Diversity & Link Relevancy - Search Engine Watch
- Wall Street Journal - As Google Tweaks Searches, Some Get Lost in the Web