There are over 100,000 gasoline stations in the US. The largest oil companies are Shell, Exxon/Mobil, Chevron, 76/Phillips, BP, Sunoco, and ARCO. Many supermarkets and club stores like Costco have their own branded gas stations. There are also smaller regional brands. You're never far from a gas station.
Gas stations quote prices in dollars per gallon and include taxes. Many gas stations charge a higher price for paying with a credit card.
Gas prices vary widely from state to state depending on taxes, regulations, season, and proximity to oil refineries. California is usually the most expensive, sometimes $1 over the national average. Texas, southern states, and midwestern states tend to have the cheapest gas. AAA publishes the current average price per gallon in every state.
Most gasoline stations sell fuel with 3 different grades. 87 is regular unleaded. 89 is mid-grade. And 91 or 93 is premium. These numbers are the octane levels. Higher octane gas is more expensive but has better engine performance and may get better mileage.
Your car's manual or fuel door will say which type of gas the engine needs. Most cars use 87 but some require higher octane.
Some stations have a fourth pump for diesel. It's a completely different fuel used mainly by big rig trucks. Only 3% of cars in the US are diesel. Its price is similar to gasoline.
Consumers can get discounts at gas station using supermarket rewards and branded credit cards. Most of the large oil companies have their own credit cards and have rewards programs with supermarket chains.
Chevron partners with Safeway/Vons/Pavilions to offer up to 20 cents off per gallon. Shell and 76 have credit cards with 5 cents off per gallon. Many regular rewards credit cards have double or triple points at gas stations.