Manual Penalties
Manual penalties are the result of Google's Web Spam team when they discover unnatural links pointing to your site. If the team believe that you have engaged or are engaging in any spammy practices outside of Google's Webmaster Guidelines you will be subjected to a manual penalty that could see your site drop of the search engine results pages altogether.Generally with manual penalties you will receive a notification of what has happened and why in Google Webmaster Tools. You can check for this warning by navigating to the Manual Actions section of the tool, under the Search Traffic subheading.
Google Penguin
If you have not received a manual penalty but have seen significant drops in search engine rankings, it is possible that your site has suffered as a result of the Google Penguin algorithm update This would still indicate spammy and unnatural links that you need to identify and deal with.Of course, there could be any number of other issues causing or contributing to your sudden loss of or drop in rankings. For more information, please check out my latest post:
Why Have My Rankings Dropped? An SEO Troubleshooting Guide