A lot of clients as me about "Google Panda", what it is and how it impacts their website. Google Panada or the Google Panda algorithm is a alteration to the Google's search results ranking algorithm that was first released in February 2011. This algorithm dubbed "Panda" after one of Google's employees who created the new filter, was the first ever penalty applied to results aimed at targetting poor content that is considered "thin" or "not good enough". Released on February 24 2011, it saw a lot of sites like content farms (where content is duplicated from site to site) fall from high ranking positions. Since February 24 2011, there have been many updates to the Panda algorithm, however it's also worthwhile noting that Google updates their algorithm up to 600 times a year. Some of these changes are minor, but every few months a major change is rolled which can impact on search results.
Google Inside Search - The Official Google Search Blog
SEO Moz - Rand Fiskin