Brent Crude is the biggest of the many major classifications of oil consisting of Brent Crude, Brent Sweet Light Crude, Oseberg, Ekofisk and Forties. Brent Crude is sourced from the North Sea. The Brent Crude oil marker is also known as Brent Blend, London Brent and Brent petroleum. It is used to price two thirds of the worlds internationally traded crude oil supplies.
The other well-known classifications (also called references or benchmarks) are the OPEC Reference Basket, Dubai Crude and West Texas Intermediate (WTI).
The name "Brent" comes from the naming policy of Shell UK Exploration and Production, operating on behalf of Exxon and Shell, which originally named all of its fields after birds (in this case the Brent Goose).
Oil production from Europe, Africa and the Middle East flowing West tends to be priced relative to this oil, i.e. it forms a benchmark. However, large parts of Europe now receive their oil from the former Soviet Union, especially through Russia.
In a move to provide traders a currency hedge, pan-Asian multi-product commodity and currency derivatives exchange Singapore Mercantile Exchange currently lists Brent Crude futures contracts denominated in Euros. Aside from traders with Euro-related business, companies with USD exposure can also diversify their currency hedging requirements.

Monthly average Brent spot prices from May 1987 – March 2009

Petroleum (L. petroleum, from Greek: petra (rock) + Latin: oleum (oil)) or crude oil is a naturally occurring, toxic, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earths surface. Petroleum is recovered mostly through oil drilling. It is refined and separated, most easily by boiling point, into a large number of consumer products, from petrol and kerosene to asphalt and chemical reagents used to make plastics and pharmaceuticals. (A better citation would be preferred here. You can help Wikipedia by providing one.)
The term petroleum was first used in the treatise De Natura Fossilium, published in 1546 by the German mineralogist Georg Bauer, also known as Georgius Agricola.. In the 19th Century, the term petroleum was frequently used to refer to mineral oils produced by distillation from mined organic solids such as cannel coal (and later oil shale), and refined oils produced from them; in the United Kingdom storage (and later transport) of these oils were regulated by a series of Petroleum Acts, from the Petroleum Act 1862 c. 66 onward.

Pumpjack pumping an oil well near Lubbock, Texas.

Natural gas is a gas consisting primarily of methane, typically with 0-20% higher hydrocarbons (primarily ethane). It is found associated with other fossil fuels, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.
Most natural gas is created by two mechanisms: biogenic and thermogenic. Biogenic gas is created by methanogenic organisms in marshes, bogs, landfills, and shallow sediments. Deeper in the earth, at greater temperature and pressure, thermogenic gas is created from buried organic material.
Before natural gas can be used as a fuel, it must undergo processing to remove almost all materials other than methane. The by-products of that processing include ethane, propane, butanes, pentanes, and higher molecular weight hydrocarbons, elemental sulfur, carbon dioxide, water vapor, and sometimes helium and nitrogen.
Natural gas is often informally referred to as simply gas, especially when compared to other energy sources such as oil or coal.

Natural gas production by countries in cubic meters per year.

Natural gas is a gas consisting primarily of methane, typically with 0-20%

 

From Wikipedia, the free encyclopedia : Extraction of crude petroleum and natural gas