Oil and Gas are lumped together partly because they are found together while drilling, but also because they can be substituted for each other in many of their main uses.
Oil and Gas have many uses. The most common use for oil is for fuel for internal combustion engines. Petrol/gasoline and diesel are the most common fuels. Oil is also refined in to plastics and for other useful industrial purposes.
Gas is mainly used for heating, although gas powered vehicles work well, and produce very little in the way of emissions.