How does the U.S. government view Bitcoin? Is it a currency, a commodity, or something else?
The U.S. government views Bitcoin as a commodity, kind of like gold.
If you buy Bitcoin at one price in dollars, and sell it at a higher price in dollars, you will have a gain (in dollars), that you will have to pay tax on.
If you buy some Bitcoin (using dollars), and then use the Bitcoin to buy a car, you have made a "two stage" purchase of a car. (If you then re-sell the car for dollars, you may have a taxable gain if the dollars you receive are more than the dollars you used to buy the Bitcoin that you used to buy the car.