Google Panada is an update to Google's Algorithm that occurred around February 24th 2011.
The initial Panda update did in fact affect approximately 12% of the websites in Google, however the effects were much larger than Google has stated because most of the sites that were affected were websites that rank in the top pages of Google
Google have publicly stated that Panda is not the only significant update that they've been running and that there will be more than 500 tweaks to their algorithms throughout the year.
