Black holes are areas of intense gravitational attraction. They form from massive stars that collapse in on themselves when they run out of fuel to 'burn'.
Stars work by converting one gas into another - usually hydrogen into helium.
Eventually most of the hydrogen is converted to helium so helium is then converted to carbon. All of these reactions release energy in the form of heat and light. It also maintains the shape of the star. Stars are made of gas. The only thing stopping gravity pull all the gas to the middle is the energy given off by these reactions.