It All Starts With Shale
Shale is a fine-grained sedimentary rock formed from the compaction of silt and mineral particles. Black shale contains organic materials that sometimes break down to form natural gas or oil. While conventional drilling can extract significant amounts of crude oil and natural gas, the majority remains trapped within tiny pore spaces or adsorbed onto clay mineral particles that make-up the shale. In the late 1980s, natural gas drilling companies developed a new technique, horizontal hydraulic fracturing, to retrieve the trapped oil and natural gas .

About Hydraulic Fracturing
Hydraulic fracturing, "a.k.a. fracking," involves the injection of large quantities of water, sand and chemicals at high pressure down deep wells to create small fractures (less than 1 mm) in shale rock formations for the purpose of releasing natural gas or crude oil held within the rock formations. Here's a map showing the known shale plays in North America.

Source: U.S. Energy Information Administration (Oct 2011)

Here's a rundown on the major shale plays.

Bakken Shale
A formation of sedimentary rocks underneath the states of Montana and North Dakota ranging in depth from 4,500 to 7,500 ft.. Estimated to contain 4.3 billion barrels of crude oil.

Barnett Shale
A 6,500 sq. mile formation of sedimentary rocks around 8,000 feet deep underneath the city of Fort Worth, Texas and surrounding areas. Estimated to contain 43.4 trillion cubic feet of natural gas.

Eagle Ford Shale
A 3,000 sq. mile formation of sedimentary rocks in southwest Texas ranging in depth from 5,700 to 10,000 ft.. Estimated to contain 21 trillion cubic feet of natural gas and 3.4 billion barrels of crude oil.

Marcellus Shale
A 95,000 sq. mile formation of sedimentary rocks underneath the states of New York, Pennsylvania,, West Virginia, Ohio and Maryland ranging in depth from 4,000 to 8,000 ft.. Estimated to contain 410 trillion cubic feet of natural gas.