Image provided with permission from the EPA site on the Hydraulic Fracturing Water Cycle. Click to view that site. This also applies to Oil extraction.
Fracking is a well completion process. After the hole is drilled deep underground, and a cement casing is installed, with other layers of steel and tubing. Any easy or conventional oil is extracted, an operator gets to choose a stimulation or completion technique. Fracking is one of those options. First, an operator generally hires a fracking service company like Halliburton or Baker Hughes. They evaluate the formation underground to determine what kind of frack fluid mixture is needed. Once that is determined, the frack job is "designed" in a way to extract the most amount of oil and gas. A lot of water (can be fresh, produced or recycled water) is mixed with sand and chemicals. The water is the "delivery vehicle" - it moves the sand down the hole. High pressures are applied and the hard rock actually breaks or fractures, the sand is pushed into these fractures and acts as a "proppant" essentially propping open the rock allowing the oil and gas to escape out into the wellbore hole. The chemicals are needed to make everything go smoothly. To lubricate, to keep things from clogging and so on. A frack job can take as little time as a half hour, up to ten or more days. A well may produce for 50 years after a frack job according to DOGGR Supervisor Tim Kustic. At some point there is produced water containing frack fluids that has come up out of the well. These need to be disposed of. While it is recognized that many of these fluids are toxic, (in fact many are carcinogenic), the entire process of fracking was exempted from the 1972 Clean Water Act in 2005 with the energy bill passed by the newly elected Bush/Cheney administration. Hydraulic fracturing is specifically called out as an exempt process. These fluids are not required to be handled in the same way as other toxic industrial wastes, and are classified as Class II Wastes.
A fracking operation can be identified by water trucks. Water must be on the premises and ready to be mixed with chemicals. Chemical & frack servicing trucks. They may be Halliburton, Baker Hughes or another "well servicing" company.