Google Penguin is the official name for Google's algorithm update planned to fight against web spam.The update was published on April 24th 2012 and affected more than 3.1% of English searches.
Penguin focuses on off-site factors of a website, rewarding those sites which have a link profile links with high quality domains and non manipulated, and trying to punish those websites who have violated Google's guidelines, that have link profile most likely unnatural, too many low quality links, etc.
If you have been penalized for updates or you want to make sure your strategy is in harmony with Google's manual, monitor your link profile to detect suspicios website's links in large amounts. After this, base your link building strategy in content, contact with quality websites and online actions. Try to avoid at all costs to generate links that can be interpreted as unnatural.