Manual Review: Manual review refers to a process major search engines like Google perform to catch search spam and train their relevancy algorithms. As the name implies, the review process is undertaken by a human member of the search engine's web spam team. If the manual review process reveals any infractions or problematic qualities, it will directly impact the ranking of a website.
While it wasn't always clear whether or not a manual review had been conducted or if a website was just being impacted by a competitor's SEO strategy, Google now reports most of their manual actions by directly contacting the webmaster to inform them of the repercussions. A number of things can attract the attention of a manual review, including abnormal usage data, link growth patterns, or spam reports. Fortunately, if a website makes an effort to correct the issues with their website, they can typically file a reconsideration request to have the penalties removed.
One of the most famous examples of the impact of a manual review came in 2011, when J.C. Penney was accused of using paid links. They were removed from prominent search ranking position for both their general and long tail keywords, a punishment that was intended to last 90 days before they submitted a request for reconsideration and were eventually returned to their original status.