Hydraulic fracturing, or "fracking," is a method used to extract otherwise inaccessible deposits of oil and natural gas by cracking the rock layers it's trapped in. In a typical fracking operation, high-pressure water, sand and slurry are pumped into a channel in the lode-bearing rock, and the pressure causes fractures.
Shale and other types of sedimentary rock often contain oil or natural gas in very small pockets deep within the stone. Fracking shatters the rock, which allows the oil or gas to seep out and be collected. Drillers do this by drilling a horizontal shaft perpendicular to the vertical well and pumping the fracking slurry into the shaft at high pressure and temperature.
The sudden increase in pressure cracks the overlying rock matrix, widening existing fissures and letting out the gas. Drillers then pump the slurry out of the shaft. This lowers pressure inside the well system and allows the deposits to flow or seep into the main drilling channel. Oil pools in the main channel, while natural gas rises. In either case, it is much easier to collect the deposits after the fracture, and subsequent seep than is possible with more conventional methods of extracting hard-to-get-at oil or natural gas.