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.
The Google Penguin Update launched on April 24. According to the Google blog, Penguin is an “important algorithm change targeted at webspam. The change will decrease rankings for sites that we believe are violating Google’s existing quality guidelines.” Google mentions that typical black hat SEO tactics like keyword stuffing (long considered webspam) would get a site in trouble, but less obvious tactics (link incorporating irrelevant outgoing links into a page of content) would also cause Penguin to flag your site. Says Google,
Sites affected by this change might not be easily recognizable as spamming without deep analysis or expertise, but the common thread is that these sites are doing much more than white hat SEO; we believe they are engaging in webspam tactics to manipulate search engine rankings.