Everything in the universe that is above absolute zero gives off heat. Only a hypothetical substance that is perfectly ordered having zero enthalpy and entropy at absolute zero and whose atoms are stationary gives off no heat.
Heat transfer through conduction, convection or radiation all result when atoms vibrate due to thermal energy. In conduction, this vibration induces vibration in nearby atoms, transferring heat to these neighboring atoms. These atoms then induce vibration in their neighboring atoms and so forth until the substance is heated through. Once all of the atoms are vibrating in sync, no more heat transfer occurs.
Convection occurs when the thermally vibrating atoms are free to move as a liquid or gas. These thermally vibrating fluid atoms collide and mix with one another causing them to exchange heat. Convection is different from conduction in that it involves mass transfer in addition to heat transfer; the heat is transferred in the movement of the atoms themselves instead of just between the fixed atoms.
Radiation occurs when the thermally vibrating atoms give off photons of energy. The more vigorously these atoms vibrate, the higher the energy of the photons they emit. Unlike conduction and convection, which require a material medium to occur, radiation can transfer heat through a vacuum.