Infrared heater bulbs screw into a socket the same way as a regular incandescent bulb. The main difference is that the infrared bulbs are made of ceramic instead of glass, which conducts heat better. On some models, the shape is similar, but the infrared bulb has a swirled top.
Infrared heater bulbs provide heat by producing light that is invisible to the eye, but that warms the skin when sitting directly in front of the device. Though they are found in portable ceramic room heaters, these bulbs are also used to provide warmth in animal cages, especially for reptiles. The infrared bulb generates heat, but there is no bright light to disturb the sleeping cycle of the animals.