Google's Panda algorithm update has affected many sites with low quality content. Google rolled out this update internationally months ago. This is one of the major Google updates in recent times. This has improved the rankings of the sites with good high quality content. With automatic softwares, many sites started scraping content from others sites & displaying it in their sites. These sites have been affected by panda update. The aim of the update was to provide high quality sites to users. Google has also published a list of checkpoints to use if the site can be penalized by the panda update.
From archive: May 2011 X