Hydraulic fracturing, a.k.a. fracking, commonly refers to the process of retrieving oil or natural gas from below the Earth's surface. Companies drill a high pressure injection of water, sand, and toxic chemicals into rock formations in order to crack the rock and release oil or natural gas trapped within.
Fracking has detrimental effects on the environment and on human health.
Fracturing rock releases hazardous gases such as hydrogen sulfide, sulfur dioxide, and methane. Exposure to these gases can cause blood disorders, negative neurological effects, breathing problems, and reporductive challenges.
The process is also no friend to water conservation. Each "frack" process uses 4 to 7 million gallons of water, and one well requires multiple fracks.
Some of the chemicals used to blast into the rock are highly toxic. These chemicals are present in the wastewater produced during fracking. Though companies store the toxic water in one place, some of it can leak into the ground. While groundwater is treated before it becomes tap water for human consumption, treatment centers typically don't have the technology to eliminate highly toxic wastes.
In the United States, oil and natural gas drillers are not required to submit which chemicals they use to regulatory agencies. There is no government oversight as to how these toxic fluids affect human health, even though there will be 32,000 active wells in America by 2012.
Progress is being made by the government, though. The U.S. Environmental Protection Agency  is currently investigating the impact of fracking on public health and water, with results being released in between 2012 and 2014.
Earth Justice