The key difference between thermal IR and the other two is that thermal IR is emitted by an object instead of reflected off it. Infrared light is emitted by an object because of what is happening at the atomic level. Atoms are constantly in motion. They continuously vibrate, move and rotate. Even the atoms that make up the chairs that we sit in are moving around.
Atoms can be in different states of excitation .
In other words, they can have different energies. If we apply a lot of energy to an atom, it can leave what is called the ground-state energy level and move to an excited level. The level of excitation depends on the amount of energy applied to the atom via heat, light or electricity. Think of the electrons in this cloud as circling the nucleus in many different orbits . Although more modern views of the atom do not depict discrete orbits for the electrons, it can be useful to think of these orbits as the different energy levels of the atom.