The Google updates Google Panda Update Overview

This update is designed to reduce rankings for low-quality sites—sites which are low-value add for users, copy content from other websites or sites that are just not very useful. At the same time, it will provide better rankings for high-quality sites—sites with original content and information such as research, in-depth reports, thoughtful analysis and so on.

Basically, Panda updates are designed to target pages that aren’t necessarily spam but aren’t great quality. This was the first ever penalty that went after “thin content,” and the sites that were hit hardest by the first Panda update were content farms (hence why it was originally called the Farmer update), where users could publish dozens of low-quality, keyword stuffed articles that offered little to no real value for the reader. Many publishers would submit the same article to a bunch of these content farms just to get extra links.

Panda is a site wide penalty, which means that if “enough” (no specific number) pages of your site were flagged for having thin content, your entire site could be penalized. Panda was also intended to stop scrappers (sites that would republish other company’s content) from outranking the original author’s content.

Here is a breakdown of all the Panda updates and their release dates. If your site’s traffic took a major hit around one of these times there is a good chance it was flagged by Panda

Source: http://www.brickmarketing.com/blog/panda-penguin-updates.htm