Basic info first: Panda was a 2011 Google algorithm update, and—cute name notwithstanding—it was one of the bloodiest updates in SEO history. (And it was named for a person, not the black-and-white bear.)
The salty SEO veterans among you might remember that day like an old war story. For the rest of you, here are two things you whippersnappers need to know about this landmark change in Google Search:
Until Google’s codename was public, SEOs called it the ‘Farmer’ update. Danny Sullivan gave it this name because one of Panda’s main effects was counteracting “content farms.” Back then, sheer keyword volume and similarly hacky tricks could still bend Google’s search results; firms could crank out tons of SEO-ready, keyword-stuffed content (with no actual value to readers) and then mint cash by displaying ads to all of the organic traffic passing through.
Google knew that this was a sub-optimal user experience at best—and at worst, it was a deadly parasite upon their business model. So the Panda update went all Third Punic War and sowed salt in the content farms’ fields.
The Panda update was designed to help search results mimic actual human preferences. It’s since become public knowledge that Google uses actual human reviewers to help them “calibrate” their algorithms for the actual people who use Google Search (but not to influence rankings directly).
There are countless technical details involved in the Panda (or any) algorithm update, but the point is that it “targeted” content farms not by maintaining some kind of blacklist, but by teaching the algorithm how to better avoid the things people naturally dislike about content-farm pages (thin content which is 99.9% useless, stuffed with ads, etc.).
To help guide webmasters, Google published this blog post with 23 self-assessment questions whose collective essence is Are you producing good, substantive content that you would trust if you were a reader?
This has been a prevailing theme in SEO for the past decade; after all, connecting users with helpful information is the human purpose of SEO. Google will always have clear incentives to strive in that direction, and over time there are fewer and fewer “loopholes” available to game the system—so there’s never been a better time to be an honest marketer competing on merit (and it feels really nice to be able to write that).
If you’d like more details, check out this guide from Search Engine Journal!