Hydraulic fracturing, or hydrofracking, is a technique in which rock is fractured by a pressurized liquid, most often to facilitate the extraction of oil and/or natural gas. A solution typically composed of water, sand and other proppants, thickening agents, and other chemicals is injected at high pressures into a well to create cracks in the deep formations through which oil and natural gas can flow more readily. The abundance of natural gas in the Marcellus shale formation has brought hydrofracking to the forefront of the American energy dialogue.
A diagram about the basics of hydrofracking. (Image source)