Sour crude is a type of unrefined oil that contains sulfur. It is difficult to refine and usually fetches a lower price.
Crude oil is considered sour when it has more than 0.5% sulfur. This and other impurites make sour crude difficult and costly to purify, so it is typically only used to produce diesel fuel rather than gasoline.
Sour crude oil is usually compared to sweet crude oil which is low in sulfur and other impurities and is easy to refine.