West Texas Intermediate is a light (40 degrees API gravity) sweet blend of crude oil produced from fields in Western Texas. It is the main benchmark for U.S. crude oil prices and is a deliverable grade for NYMEX crude oil futures contracts. A crude with more than 0.5 percent by weight of sulphur is a sour crude, with less than 0.5 percent sulphur it is said to be sweet.

See also: Brent, Dubai Crude, http://www.cmegroup.com/trading/energy/crude-oil/light-sweet-crude.html