A Google Penalty (or manual review) is the negative impact on search rankings and can range anywhere from a noticeable decrease in visibility to having pages deindexed temporarily until the issue is resolved. These drops in rankings can occur on a specific page, a sub domain, or an entire web property. Sometimes they are based on changes in algorithm updates, as well as manual inspection from a Google web spam team member. Typically Google penalizes websites that have attempted to manipulate search results using links or other resources. These resources include:
Guest Blog Posts
Example of “Thin Content” Penalty (Panda type)
You can overcome a Google Penalty by filing a reconsideration request using Google Search Console, and documenting the steps taken to clean up the penalty.