Thermal Radiation is heat transfer via the use of electromagnetic waves such as infrared light, visible light, radio waves, microwaves, x-rays, and gamma waves. This transfer of heat is done without the use of another mechanism.
The Sun is the most important source, but everything that is above 0 K or absolute Kelvin emits radiation. This includes books, ice, plants, and concrete. The Sun itself emits light in the visible, infrared, and ultraviolet ranges.
Infrared waves are heat waves, and they are of extremely high frequencies. These frequencies are higher than that of radio waves, but less than that of visible light. They also have wavelengths longer than visible light, with lengths from 7 x 10-7 m to 1 mm. When objects absorb energy from these waves, this causes the atoms inside of them to vibrate and produce heat.