Last week Expedia was hit with a 25% drop in search results for some of their most prominent keywords on Google. This was because of what Google deemed unnatural links. Similar to what happened to Rap Genius in December of 2013.
Unnatural links are artificial links such as purchased links or links in a bad neighborhood of the internet that point to your site. Examples of bad neighborhood sites include scraper sites (sites that copy all of its content from other websites), link farms (a group of sites that all link to one another) , sites with large amounts of spam, malware or illegal or pornographic material.