Unlike Conduction and Convection, Thermal Radiation is transferred by waves not particles because of this Thermal Radiation is the only heat transfer which can work inside a vaccum(a space with no matter (therefore no particles for the convection and conduction to use)).
Radiation is also known as Infra-Red Radiation and is a type electromagnetic radiation. All objects are continuously emitting and absorbing thermal radiation and the hotter the object the more thermal radiation is emits and the less it absorbs in contrast the cooler an object is the more thermal radiation it will absorb and the less it will emit.
Dark, dull or matt surfaces are better absorbers and emitters of thermal radiation.
Also the bigger the surface area the faster an object radiates thermal radiation because the bigger the surface area the more waves can be emitted from the surface therefore the transfer of heat is faster. (this is also useful in biology because animals with a larger surface area radiate more heat and lose heat faster than more compact animals)