Hydrofracking is an industry shorthand for hydrofracturing. Why didn't I node this under that title? Because the word frack is too awesome to pass up. And no, you don't do it in a spacefaring hot tub.
The Federal Remediation Technologies Roundtable website offers a description of hydrofracking. According to them, "Hydrofracturing is a pilot-scale technology in which pressurized water is injected to increase the permeability of consolidated material or relatively impermeable unconsolidated material." Nice. What?
Hydrofracturing is a means of increasing the flow of water into a bedrock well. Bedrock wells are either plain shafts or (usually) a shaft which travels through several cracks in the surrounding bedrock. Water seeps into the cracks all along their length, and ends up in the well itself. If a bedrock well has inadequate flow (doh!), one means of improving it is to drill deeper. However, if that is not feasible, hydrofracking may be the answer. In this process, water is pumped into the well from the top at extremely high pressure. The pressure and, sometimes, the flow of water into the well will either force the cracks in the bedrock to expand, and/or will erode the edges of the fissures, making them wider. If the well has become fouled by debris clogging the fractures or main shaft, the pressure and erosion will serve to clear away this debris. Once this has been done, the pressure is removed. If the process has been successful, the well will see an improved flow rate.
As a remediation technology, hydrofracking is sometimes used with abrasives such as sand in the pressurized water stream to increase the erosion of the fracture edges. High-tech variants will include binding agents such as guar gum intended to increase the viscosity of the pressurized injection and 'trap' clean sand particles inside the fractures under pressure. Once that process is complete, the binding agent is dissolved using a solvent, leaving the rock and sand matrix behind to hold the fractures open, but porous enough to pass water (har har).
Google 'hydrofracking' and (if you're in the US) you'll probably come up with a host of local well-drilling companies.
As of early 2011, the process is mired in controversy over the use of various chemicals in the injection process - evidence is being presented that known carcinogens are being used in close proximity to drinking water wells, and turning up in their output.