There are 3 ways that heat energy travels - conduction, convection, and radiation. Conduction is when you directly touch a hot object and the heat energy moves directly into the cooler body. Convection is when an intermediate fluid such as air or water carries the energy from hot to cold. The third way is by Radiation. All objects with a temperature above absolute zero radiate heat. This radiation works very similarly to visible light - warm objects glow - but room temperature objects glow deep in the infra-red portion of the spectrum. The hotter the object is, the higher the peak frequency of the radiation. Human eyes can't see infra-red light, so we can't see objects glowing with warmth until they get hot enough to glow red hot (or hotter). Humans do have a sensor that can detect some infrared radiation - our skin! If the radiation is strong enough, you'll feel the heat. Thermal imaging devices CAN see infrared radiation. They focus and convert this energy into visible light that we can see basically by using an infra-red sensitive tv camera and display. One small problem is the camera can't see objects that are colder than room temperature because the camera itself is warm and emits heat that blinds itself. To get around this, many thermal cameras are cooled using electronic coolers or cold refrigerated liquids like liquid nitrogen or helium to make them more sensitive.