Google's Phantom Update was a sneaky change to the company's search algorithm that affected mostly informational sites like eHow, Quora, and HubPages. Many sites lost rankings while others jumped ahead, causing mass confusion among webmasters as Google originally denied a major update.
The major reason sites dropped in the rankings was because they were determined by Google to be low-quality. This is causing many site owners to adapt their sites to Google's definition of quality in order to improve the user experience and regain rankings.
You can read more about what was specifically impacted here: Don't Blame Google's Phantom Update For Your Lost Site Traffic