The basics of the classifications defines the American Petroleum Gravity (API), which marks the gravity and viscosity of the material. The most important properties are the proportion of hydrocarbon elements and sulfur content. This two, together with the API gravity, decides if a type is heavy, light or intermediate. In relation with sulphur oil can be sweet (low content of sulphur) or sour.
Brent Crude and WTI are two of the major Classifications of Crude. Brent Crude is extracted in the North Sea. It is one of the lightest and sweetest crude oil types, with an API gravity of 38.3 and 0.37% of sulfur. It's price actually (May – June 2011) fluctuates between $115-110.
Very similar, to Brent Crude, in composition and API gravity, is the WTI, West Texas Intermediate, which is gained in the United States rather on the Gulf coasts. Although it's similar to Brent, its price is more lower, $95-100.
This two types build the most important western oil types, while there are other well known classifications, as Bonny light from Nigeria, Basra light from Iraq, Arab light from Saudi Arabia (that type with the greatest reserves) or even Saharan blend (Algeria) and Minas (Indonesia).
The types found near to the earth crust are easier and cheaper to extract and refine, although there are less discovery points as for the heavier and sourer oils. That's why their price is always higher and the fluctuation a very important factor for the buyer and seller.