I was asked recently to explain the difference between these two algorithm changes made by Google in 2011 and 2012 respectively.
Panda, named after a Google engineer, was designed to root out real content. It was no longer enough to stuff a bunch of keywords onto a page to achieve page one ranking. Google got smarter about checking the rest of the page and the site for real relevant content.
Penguin on the other hand went looking for backlinks or inbound links and measured the quality of the domain that they came from and what the links actually said about the site they were linking to.
The double whammy of these two changes coming within a year of each other toppled many a website that had enjoyed page one dominance, especially those which had used somewhat underhand tactics to get there.
I was lucky enough at the time of these seachanges to be working for a web agency that had always practiced what they preached in terms of SEO and their websites, while not unaffected, benefited from a solid structure and optimisation advice.
Hummingbird – a total rewrite of Google
The next update to happen along caused less ruffled feathers than the previous two but was in fact even bigger. Rather than a change in algorithm, Hummingbird was an overhaul of the whole, keeping the engine humming along (pardon the pun). It makes Google much smarter at figuring out who in the internet world is a genuine supplier of the services or goods described.
Update: In case you didn’t know Panda, now on update 28, and Penguin are still rolling out today.