About 1 year ago Google implemented its “Panda Update” to clean up their search results and get rid of low quality and thin content. They want higher quality content with higher user engagement at the top of their search results. I have posted an Infographic written by Search Engine Land designed by Blueglass that shows the timeline over view of the Panda 1.0 – 3.2 Updates.
I would recommend reading the Blueglass blog along with Search engine land for more in depth analyses of the Panda Update.