Google made a core algorithm update over the weekend that caused SEOs to lose their minds. It's not fully understood yet but the story has been confirmed by Google through Search Engine Land.
Furthermore, and seperately, it seems that Panda is now a part of Google's core ranking algorithm. What does that mean? Well, for one thing, it means that Google will probably never confirm another Panda update again. SEL says that the algorithm likely isn't updating in real time. Gary Illyes confirmed as much on Twitter.
And if you have any more questions, Jennifer Slegg at The SEM Post did an absolutely marvellous and comprehensive write-up of the new Panda.