Shale Gas is methane (natural gas) which is trapped in impermeable shale rock deep underground, unlike conventional natural gas which is in permeable rocks, such as sandstone.
The gas cannot flow through the shale, so simply drilling a well, as you would for conventional natural gas, is not enough. The shale rock must be cracked to free the gas, hence the need for hydraulic fracturing (fracking).
For the same reason it is necessary to drill large numbers of wells at regular intervals. To produce as much gas as a conventional gas field with a dozen or so wells, would require hundreds or thousands of shale gas wells.
Because of the intense nature of the shale gas extraction process, it is associated with much more negative impacts than conventional drilling. These include leaking methane, water contamination, air pollution, radioactive contamination, massive industrialisation of the landscape, worsening climate change and earthquakes.
Severe health effects in people and animals are beginning to mount areas where shale gas extraction is widespread. Shale gas extraction also leaks large amounts of methane (a very strong greenhouse gas) and makes available fossil fuels that would not otherwise be burnt, both significantly worsening climate change.