This radiation can be produced by electric currents or other charge currents moving the right way, energy from excited states in molecules and atoms, thermal radiation, etc.
Thermal radiation given off by all objects that can absorb some form of light. The total energy given off is proportional to the fourth power of the temperature from absolute zero, the area, and the amount the object absorbs light. The radiation has a shape kind of similar to a bell curve, with the peak frequency proportional to the temperature (so hotter objects glow infrared, red, yellow, etc. as the peak frquency gets higher and higher.), but all radiation is given out in higher amounts at higher temperatures. Sometimes, the fraction of incoming radiation absorbed/emitted changes with frequency, and some absorption is due to quantum effects, molecular excitations, radio receivers, chemical reactions, etc. rather than directly heating up the object.
Energy from the sun is mostly coming in as visible light, as the peak radiation from the sun comes in this area of the spectrum, but at most earth temperatures the radiation will leave as infrared. Because the energy coming in from the sun balances with energy leaving the earth, it means more energy is leaving as infrared, but coming in as visible. The greenhouse gases absorb infrared radiation much more effectively than visible light.