The price of crude oil refers to the spot price of either WTI/Light Crude on the New York Mercantile Exchange (NYMEX), or Brent as traded on the Intercontinental Exchange (ICE). The price of a specific barrel depends mostly on its grade (determined by sulphur content and specific gravity) and location.
Oil's price is mostly affected by supply and demand, however it can also be affected by speculation and uncertainty (particularly when events may threaten large producing areas or pipelines). Demand is highly dependent on global economic conditions, with high growth pushing up oil prices, but high oil prices restricting growth. Supply can be affected by events such as Hurricane Katrina or the War on Terror, and by the actions of suppliers. OPEC, which consists of Algeria, Angola, Ecuador, Iran, Iraq, Kuwait, Libya, Nigeria, Qatar, Saudi Arabia, the United Arab Emirates, and Venezuela, controls some 75% of the World's oil reserves, and was set up to allows its members to control the price of oil, leading some to accuse it of operating like a cartel. It is interesting to note, however, that Canada, has the the second largest oil reserves in the world (considerably more than third place Iran) and the United States is 11th, just ahead of Mexico.