Google Panda is a modification to PageRank that was released in February of 2011. Its main goal was to de-rank low-quality or "thin" sites, thus bring higher-quality information higher up in the ranks. It also brought down some sites with high advertising. It was fairly successful, and on April 24th, 2012, it was updated with Google Penguin.
Things that made it different from previous ranking systems similar to it were the facts that Google Panda affects entire sites instead of just individual pages, and that a "over-optimization penalty" has been deployed. Google Panda also ranked sites in a more scalable way than previous human-controlled systems.