Oil is formed deep within Earth, underwater. Oil began its existence as microscopic sea vegetation.
When the plants died and were covered by sediments, they were squeezed by the great force of pressure and became oil. Oil may come to Earth’s surface on its own as a result of underground activity, or it can be extracted with drills and pumps, which is more common.
We call oil a fossil fuel because it comes from fossilized plant life and we use it for energy.
Heaters, cars, and factories, among a huge number of other industrial-age inventions, depend on oil and other fossil fuels for energy.