Hydraulic Fracturing, also known as fracking, is the process of removing shale oil and natural gas from shale rock formations underground through the use of drills and the injection of a high-powered liquid solution. The process is used to harvest deposits that previously could not be exploited. The exact method of drilling and extraction varies depending on the shale deposit at issue, although all methods include drilling and injection. The process was first used in the late 1940s. Fracking is a controversial process. Although fracking would allow the United States to tap into a wealth of previously unexploited oil and natural gas, the chemicals used in the process have resulted in the contamination of the water table in areas where fracking has taken place.
For a complete explanation of the process you can visit the EPA's page.