A visual glance at energy terms and technologies

NEW YORK (AP) -- Here's a look at some key energy terms and technologies.

HORIZONTAL DRILLING: Companies used to drill wells straight down into the earth to tap pools of oil and gas that had formed over millions of years. Now they can drill down and then change the angle to follow thin layers of source rock, reaching more oil and gas with each well.

SOURCE ROCK: Wide, thin layers of sedimentary rock, like frosting in the middle of a layer cake, that are interspersed with oil and gas. In the past, drillers had to look for places where oil and gas had seeped out of this rock and into large pools that were easy to tap. Now drillers can extract oil and gas directly from source rock, opening up vast new resources.

FRACKING: The colloquial name for hydraulic fracturing. This is the practice of injecting water, sand and chemicals into source rock to crack it and create escape routes for oil and gas. Its increased use has raised concerns that the chemicals used could seep into groundwater, either through faulty wells or if it is not disposed of properly.

DOWN-HOLE SENSORS: Engineers have developed increasingly sophisticated sensors that follow drill bits and measure physical characteristics of the rocks and fluids underground. The information can be sent via fiber optic cable to engineers at the surface.

REMOTE DRILLING: Using computers, engineers can direct drill bits from command centers thousands of miles away. This reduces drilling costs.

WALKING RIGS: In the past, when rigs were finished drilling a well, they had to be disassembled and trucked to the next location, a process that took several days. Now some rigs can "walk" hundreds of feet on hydraulic shoes to the next drilling spot. This reduces drilling costs.

BIOFUELS: Liquid fuels such as ethanol and biodiesel made from plants. Biofuels made from corn are common. "Advanced biofuels" made from plant waste or algae are more environmentally beneficial, but companies have not been able to produce them at reasonable costs despite big subsidies.

RENEWABLE ENERGY: Energy derived from renewable sources such as wind, solar, geothermal or plant matter. This type of energy is growing fast in the U.S., but it remains a small contributor to the nation's energy mix. Ten percent of U.S. gasoline demand was met with biofuels and 4.8 percent of the nation's electricity was generated by wind, solar and geothermal last year.

CLEAN ENERGY: Energy that pollutes less than coal and oil, the dominant sources of fuel for electricity and transportation. Natural gas is considered by some, including the Obama administration, to be clean because it emits far fewer pollutants than coal or oil. Nuclear power is also considered clean by some, because nuclear plants emit no greenhouse gas or other pollutants into the air. Others consider only renewable energy truly clean.