These types of heaters use various heating elements to heat the air. As the air warms, it rises and is replaced by cooler air from below the element. The cool air heats up and the cycle continues. These heaters are quiet because they don't use a fan to push the hot air into the room and generate even, consistent heat. With large heating surfaces, these heaters are most often used for steady, whole room heat.
For information on Heater safety visit the Association of Home Appliance Manufacturers website.