All substances are made of small particles called atoms. Some atoms join together to make bigger particles called molecules. Water is an example of a molecule, it is made from atoms of Hydrogen and Oxygen. When molecules are formed, in most cases, they store energy, this type of stored energy is called chemical energy.
Remember the description of energy. If something has energy it has the capabilty of doing something to something else.
The molecules which make up wood are very large so they have a lot of stored chemical energy. When they break up they give out a lot of energy, some in the form of light and lots more in the form of heat energy.
Heat energy is slightly different but it is still about molecules. When we warm air or water or any other substance we make the molecules that make up the substance move faster, The hotter they are the faster they move. When these fast moving particles hit another material (your finger, maybe) it makes the molecules in that material move faster and faster. These can be moving so fast that they begin to damage the cells in your finger…..and burn.
The fast moving particles (with what is called Kinetic Energy – energy of movement) give up their energy to the finger.