Tractor is correct, USO is an ETF that tries to match the price changes of the West Texas Intermediate Crude spot price. They do it by buying futures contracts, other types of oil, natural gas, gasoline, etc...
Here are a couple of good explanatory links for you.
http://www.investopedia.com/terms/e/etf.asp
http://www.nasdaq.com/etfs/what-are-ETFs.aspx