As I see, you've heard about the google panda update... This maybe is the biggest update, made by the search engine lately. There were many websites banned, because of their unoriginal content. This is one of the main criteria - unique content...
Have a look at this article, I think it is really good explained.
www(dot)seomoz(dot)org/blog/beat-google-panda - I know it's a bit long and maybe a little bit boring, but is really useful
P.S. I am new to the forum and can't post links, so just replace the (dot) with .