Two objects will exchange radiation until they come to an equilibrium temperature. The cold object will receive photons emitted from the hot object. These will force the cold object’s electrons into motion and transfer the energy to the atoms raising their temperature. In a blackbody absorber the electrons are free to absorb any photon independent of its frequency. However in real materials some photons are reflected. If this reflected amount is equal for all photons, independent of photon wavelength, it is described by a ratio called the emissivity, with value between 0 and 1. This process is reversible, so a good absorber is a good emitter and vice versa.

A blackbody has an emissivity of 1. This means that every photon that strikes it is absorbed and conversely every photon that is generated by it, is radiated. The opposite of a blackbody is a mirror, it has an emissivity of 0. Every photon that strikes it is reflected, and every photon generated by it is retained (not emitted)

A thermos bottle (Dewar) works on this principal. It consists of a glass vessel coated by a mirror and suspended in a vacuum by a narrow neck. Because of the mirror coating there is no radiative path to cool the bottle, emissivity is 0. The only thermal path out of the vessel is thru it’s neck, which is kept small. When filled with hot liquid it will hold its temperature for many days. (It is said that on a warm desert night, water left in open Dewar will freeze solid. This is because the water, isolated from the local environment by the Dewar, has radiatively coupled to the cold night sky.)

A greybody has an emissivity between 0 and 1. The emissivity ratio can change with wavelength. In the visible regime this gives rise to the perception of color. In the infrared this is described by specta, and can be commonly used to identify chemical make up of materials.