Google changes its algorithms over 300 times per year. However big algorithm updates are usually given a name. Panda originally happened in Feb '11 but the algorithm is still run every month or so.
However the new algorithm change is called Penguin, and that's what people are more worried about now.
The original panda saw many users lose between 50-80% of their traffic - however HP brought in stricter rules and subdomains and *most* people regained their traffic and earnings.