The Penguin update has been described by Google themselves as an algorithm change that’s aimed at webspam and, more specifically, “sites that we believe are violating Google’s quality guidelines.”
Here is a snippet from Google’s Webmaster Blog about the Penguin update:
The change will decrease rankings for sites that we believe are violating Google’s existing quality guidelines. We’ve always targeted webspam in our rankings, and this algorithm represents another improvement in our efforts to reduce webspam and promote high quality content. While we can’t divulge specific signals because we don’t want to give people a way to game our search results and worsen the experience for users, our advice for webmasters is to focus on creating high quality sites that create a good user experience and employ white hat SEO methods instead of engaging in aggressive webspam tactics.