Google makes over 500 changes to their ranking algorithm every year. Many of these are very minor and almost invisible, but occasionally Google makes major algorithm updates or supplemental algorithms that have a major impact on rankings and can cause a well-ranking site to vanish from the top search engine results.
Before Google Penguin, Google released a series of algorithm updates that is called Google Panda. Google Panda first appeared in February 2011. Panda aimed at down ranking websites that provided poor user experience.