What is the main source of energy in stars?

Nuclear fusion. The particular type of fusion depends on the temperature, which is dictated by the mass and age of the star in question. For stars on the main sequence, the primary source is fusion hydrogen into helium via what is called the p-p (proton-proton) chain. For higher mass stars, they fuse hydrogen into helium primarily via the CNO (carbon-nitrogen-oxygen) cycle. As stars age off the main sequence, they can burn additional elements (such as helium, oxygen, silicone, etc), but these stages are much shorter.