What is Thermal radiation, absorptivity, black body?

Every body at temperature $T$ emits electromagnetic radiation from the thermal agitation of its charges: its thermal radiation. Its surface also receives radiation from its surroundings and absorbs a fraction $\alpha$ of it (the absorptivity, between $0$ and $1$, in general depending on the wavelength), reflecting or transmitting the rest. A black body absorbs everything ($\alpha = 1$ at every wavelength). A small hole in the wall of a closed cavity is the practical black body: radiation entering it is absorbed at the walls before it can find the way out.