Fossil fuels are a huge fraction of where we get energy. Some of these are used at power plants to make electricity (mainly coal and natural gas) or to power something like a car directly (mainly petroleum). Where does the energy come from in fossil fuels, and why do we use them so much?
Petroleum is a mixture of organic compounds, which are molecules based on carbon. Take octane as an example, shown above. Oct means eight, and octane is a molecule with 8 carbons. (Sometimes people get tired of drawing all the hydrogens in organic molecules, so they draw them as simple stick figures, with corners signifying carbons and the hydrogens implied but not drawn.) Coal (a solid) is mostly longer molecules, and natural gas is shorter molecules.
The energy in fossil fuels is stored in the bonds between the atoms. If you break these bonds, energy comes out. So we break them, and this is how the internal combustion engines in our cars run, as well as the external combustion engines in our power plants.