WTI stands for West Texas Intermediate (occasionally called Texas Light Sweet), an oil benchmark that is central to commodities trading. It is one of the three major oil benchmarks used in trading, the others being Brent crude and Dubai/Oman.
Because the standard and eventual uses of oil being drilled depends heavily on the field it comes from, oil is traded in benchmarks. These allow oil traders to know what type of oil is traded (and will eventually be delivered) immediately.
WTI is a light, sweet variety of oil. That means that it has a low density and low sulphur content, making it easy to refine. It is the second-most-traded oil benchmark, behind Brent crude, and is traded on the New York Mercantile Exchange.
The price of WTI tends to move in line with the price of Brent crude, although different global events will cause the value of each commodity to differ.