2011 Googles Algorithm Update

This update is calculated to decrease rankings for low-quality sites—sites which are low-value add for users, copy content from other sites that are just not very helpful. At the same time, it will offer better rankings for high-quality sites—sites with unique content and information such as investigate, in-depth reports, thoughtful examination and so on.

for more
http://googleblog.blogspot.com/2011/02/finding-more-high-quality-sites-in.html