Our goal is simple: to give people the most relevant answers to their queries as quickly as possible. This requires constant tuning of our algorithms, as new content—both good and bad—comes online all the time.
Many of the changes we make are so subtle that very few people notice them. But in the last day or so we launched a pretty big algorithmic improvement to our ranking—a change that noticeably impacts 11.8% of our queries—and we wanted to let people know what’s going on. This update is designed to reduce rankings for low-quality sites—sites which are low-value add for users, copy content from other websites or sites that are just not very useful. At the same time, it will provide better rankings for high-quality sites—sites with original content and information such as research, in-depth reports, thoughtful analysis and so on.
» via The Official Google Blog