Google is interested in providing the best possible results to searchers. Their mission statement reads … Google’s mission is to organise the world’s information and make it universally accessible and useful.
The updates to search results that Google has made under the name of “Panda” were released were a lot to do with the content on websites. Websites that had duplicate content to other sources on the web were hit very hard. It’s now extremely important to maintain unique page content on a website for it to do well in results.
Penguin from test so far, seems to be analysing the links back to a website. If there’s an unnaturally high number of links that contain the keyword phrase that a page is trying to rank for, this is now penalising the page.
This video at SEOmoz covers the Penguin update pretty well …
They also provide a great summary and history of changes Google make here …