Crude oil is a naturally occurring mixture of liquid hydrocarbons. In its natural state, crude oil has few direct uses. However, when it is processed through an oil refinery it can be transformed into a wide variety of highly-valued liquid petroleum products such as gasoline and diesel. The value of these products comes from their high energy density and liquid state, which makes them ideal as transportation fuels.

Crude oil comes in hundreds of different varieties, called crude grades. As a naturally occurring, raw commodity, crude oil from different fields and reservoirs can have very different properties. Crude grades from the same location, with similar properties will typically be referred to as a single grade. Examples of some well known crude grades are Brent, Tapis, and WTI.

There are a wide variety of different properties that are used to distinguish between crude grades. These are detailed in a chemical analysis of the crude called a crude assay.

The most common characteristics used to identify the quality of a crude are its API gravity and its sulfur content. The highest valued crude grades are typically those with high API gravity and low sulfur content. These are referred to as "light-sweet crudes". At the opposite end of the spectrum are the grades with low API gravity and high sulfur content, which are referred to as "heavy-sour crudes".