Learn More about Google Panda by Matt Cutts.

1. Google Panda Algorithm
 Google Panda Algorithm first released in Feb 2011.
 It affects directly to the website’s page rank based on quality of that website &
its content.
Page rank is used by Google to determine which webpage is more relevant to
search query.
Main aim of this algorithm was to lower the page rank of “low quality websites”
or thin websites” so that high quality websites can get good page rank & can
come on higher position.
 It affects entire website rather than individual webpage.