Fracking is short for "hydraulic fracturing," and the catch-all term used to describe the process of extracting oil and natural gas from shale rock formations deep underground. The process goes roughly like this: A company drills down more than a mile deep into the shale rock formations. Then comes what is known as "horizontal drilling" - effectively, the drilling turns 90 degrees, so that the well is exposed to more rock than it would be otherwise.
After the well is reinforced with concrete, tubes with perforating guns are sent down to set off explosions that create perforations in the rock surrounding the well. Then millions of gallons of "fracking fluid" - a mixture of water, sand and chemicals - is pumped down into the well at high pressure. The pressure builds up in the well, and the rock fractures. That frees the oil and gas that had been trapped within the rock, which flows back up through the pipe to be captured above ground. The process is repeated multiple times, with much - though not all - of the fracking fluid brought back up through the well.