Google's "Panda Update" was designed to filter low quality or "thin" content from its top search results. There have been several updates to Panda since it's release.
Below is a really nicely done infographic produced by Search Engine Land in conjunction with BlueGlass. It's posted here using the embedd tag they provide for sharing the graphic. It covers how Panda works, what it impacted and the various updates from Panda 1.0 through Panda 3.2 that have happened along the way.