The arterial blood
of the world economy
, this lightest of the crude oils
is the most desirable because it requires less processing
to turn into gasoline
"Light" refers to the specific gravity of the oil. A light crude has a specific gravity of 34+.
"Sweet" refers to the sulfur content of the oil. A sweet crude has a low sulfur content.
In the past, refineries were set up to work with only one type of crude. One of the main reasons the U.S. was caught short by OPEC in the 1970's was simply because our refineries depended on imported light, sweet crudes. And at the time, the unlimited source was from OPEC countries. So when OPEC cut off the flow of oil, U.S. refiners had no real alternatives. Although there was heavy, high sulfur oil available from various other sources, our refineries could not process those crudes.
In the 1980's refineries were modified to allow the use of a wider range of crude oil, which opened up the market to many new producers. Now, refiners can accomodate the heavier crude oils, but when supply is plentiful, they will always prefer the light, sweet crude.