What is Google's Panda Update?
"Panda" is the codename given to a important Google algorithm update that rolled out in February 2011.
Google recognized the growing problem with "spam" on the web, Panda's main goal was to locate websites that consistently put out weak, low-quality content and essentially weed them out of the search results.
By Google algorithm change standards, Panda's impact was huge, initally affecting 12% of websites including About.com, Associated Content, Suite 101, and Demand Media. Since then, Google has released 20 smaller updates to Panda each touching between 1-5% of search results.
And this isn't something that will pass - Panda refreshes will come down the line on a regular basis - roughly every month according to Google.
How do I know if my website has been hit by Panda?
You'll know. Panda isn't a page-by-page penalty, but something that will impact your entire site. If your site has a high ratio of low-quality pages, your site will be impacted by Panda. You'll see a major dip in your organic search as most of your pages will have dropped out of the SERP - though some high-quality pages may remain (and rank high).
What to do if your site has been hit by Panda
Reevaluate the content on the site. Was it created with the SEO in mind? Would users interested in that subject get any real value of it?
There really isn't one catch-all fix to the issue - if you want to avoid being hit (again) by Panda - then you'll need to make sure you've created valuable content. If you'd been hit by Panda, ask yourself these 23 questions to see if your site "counts as a high-quality site" provided by Google.
With already 20 refreshes to Panda, Google is seeing the results they hoped and will continue improving and recalculating Panda so that search results are relevant and valuable to the end-user. Google's search algorithm is, by far, the most advanced crawler out there, and with huge volumes of data and the ability to seek out patterns, sites with low quality content are not going to last very long.
If your website has thin, low-quality or duplicate content - you're subject to Google's Panda updates. With promises of monthly refreshes, ensure that you're providing valuable content to users so that your website isn't in Google's crosshairs.
This is a multi-part series based off the 2012 Dreamforce session "21 Changes that Impacted SEO Forever in 2012". This topic was originally covered by Adam Audette of RKG. Our session covered 21 points in an hour, so I wanted to take the time to further explain the topics in more detail over the next several weeks. Follow me on Twitter @citiseo.