Penguin

With the release of Penguin,  April 2012 which was originally called webspam algorithm update,  Google continues in their goal to help users find sites that provide good user experience. Penguin penalizes site that violated Google's quality guidelines.

sites affected by this change might not be easily recognizable as spamming without deep analysis or expertise, but the common thread is that these sites are doing much more than white hat SEO; we believe they are engaging in webspam tactics to manipulate search engine rankings
Another Step to Reward High-Quality Sites Matt Cutts, Google

Although the exact elements of Penguin are another of Google's secrets, it is affected by webspam via keywords and links.

• Keyword Stuffing
• Too many links per page
• Outgoing links that do not relate the other information on the page
• Links that do not match anchor text
• Links to questionable sites (sites that Google finds problematic)
• Links to a Malware Site (visitors to a Malware Site normally get a warning in the browser before the browser loads the site)