What is a Google penalty?
Their motivation to serve the best results is reflected in constant tweaking of the algorithm which controls which websites are prominently positioned, and which ones aren't.
The algorithm is highly sophisticated, with over 200 different ranking factors. In recent years algorithmic and manual penalties have been issued to websites who Google deem to have been partaking in manipulative practices. These penalties reduce the prominence of these sites in search results, sometimes removing them from the index altogether.