The focus of Google's Penguin update was to target webspam tactics that were still working after previous updates. Penguin affected 3.1% of English language search results, but was only intended to eliminate a small percentage of websites that were benefitting from spammy techniques. Rob Kerry explained this further in a Whiteboard Friday video for SEOmoz.
Websites that have maintained a natural backlink profile and have invested in well-written, user-focussed content.
Sites that have previously built too many links using the same commercial anchor text - creating a very unnatural looking backlink profile, as well as keyword stuffing and other on-page over-optimisation techniques.