The Panda algorithm, originally rolled out by Google in Feb. 2011, demotes the rank of websites with poor-quality content or websites that are trying to manipulate Google's ranking system. Panda affects the rank of entire websites, not individual pages. In March 2013, Google announced that Panda had become part of its core algorithm and that updates to it would be continuous. Sites that have had their rank lowered by Panda can recover if they correct the problems that got them penalized in the first place.
The history of the Google Panda update