Both updates were in fact rolled out in several phases. The first Panda update, which affected around 12% of Google searches, was issued in February 2011 and the latest in July 2013. Both updates target sites that breach Google's quality guidelines – in other words, sites that spam – but in different ways.
Panda focussed on site content and how Google evaluates it. It rewards sites with unique and high value content, and demotes sites that did not provide useful original content or otherwise add much value. The main casualties were article libraries; sites duplicated on multiple domains; 'scraper' websites containing content lifted from other pages; and sites composed of 'thin' content or mainly advertising, which added little value of their own.
Penguin, on the other hand, focussed on the quality of backlinks. It penalised 'black hat' optimisation methods which tried to trick Google into giving unreasonably high rankings. This mainly affected sites using 'doorway' pages, which exist for no other reason than to attract search engines; and pages whose backlinks mostly contained the same phrases, or came from link farms or other poor quality sites.
In both cases some legitimate sites were badly affected, particularly where the search engine failed to discern the legitimate author of duplicated or stolen content.
If your site was affected and has failed to recover over time, there are some things you can to help.
Remove and Disavow bad links. If you have inbound links from link farms or similar sites, log in to your account and remove them, or email the webmaster and ask to have them removed. Google also provides a page in Webmaster Tools where you can ‘disavow’ such links. You should do this for all links that you cannot get removed. For those you can get removed, it is probably worth doing anyway in the meantime: https://www.google.com/webmasters/tools/disavow-links-main?pli=1
Fix duplicate content. If you have identical content on multiple pages, either remove all but your preferred page or add rel=canonical links to the <head> section of the others. This tag tells Google which page you consider to be the original and would like to be indexed. The format is:
<link rel="canonical" href="http://www.mysite.com/preferred-page.html"/>
Improve poor content. Boost your site's quality, and potentially its rankings, by improving the quality of its content. Remove needless waffle, beef up or remove pages with minimal content, and re-write content that was taken from elsewhere. Google provides a useful checklist for improving poor-quality content, http://googlewebmastercentral.blogspot.co.uk/2011/05/more-guidance-on-building-high-quality.html
Plan a compliant link strategy. Good quality links still matter. Look to business partners and high-value non-commercial sites in your general market for relevant reciprocal linking. Social networking, forums and blogs are still invaluable sources. Links from sites in your own locality should help your rankings in local search.
Avoid sites and strategies that breach Google's guidelines:
Plan a compliant content strategy. Follow Google's guidelines when developing future content for your site. These are outlined here: https://support.google.com/webmasters/answer/35769
Avoid 'black-hat' optimisation methods and content sources that breach Google's quality guidelines:
In general, think less about how you can achieve good search engine rankings. Think more about how you can provide high quality and worthwhile content for your visitors, and how you can attract more visitors to it.
By Bruce Townsend - Sellerdeck