Heat

Heat is the energy an object has because of the movement of its particles, atoms and molecules. These are continuously jiggling and moving around, hitting each other and other objects. When we add energy to an object, by burning it or passing electricity through it, its atoms move faster and it gets hotter.

Hot objects can emit radiation. This happens when high energy electrons in an atom levels fall down to a lower energy. We see this radiation as light.

Temperature is a measurement of the heat in an object - the faster its atoms are moving the higher its temperature. Thermodynamics is the branch of physics which studies the movement of heat.