Infrared radiation is an invisible form of light that we usually detect as heat, like the sun shining on our face, or the warmth of a campfire. It has all the same properties as visible light: for example, it can be focused and reflected. The only difference is that it has a longer wavelength, which means we can't see it with the naked eye. Light is made of tiny particles called photons, and the wavelength tells us how fast those particles are vibrating. The shorter the wavelength, the faster the particles are moving. Shorter light waves look blue, and longer ones look red. The wavelength of infrared light is so long that we can't see it at all.
Infrared light is increasingly being used as a form of therapy for everything from arthritis pain and skin abrasions, to varicose veins and cellulite reduction. This is because it can activate enzymes in the human body, which then trigger other responses. For example, infrared lasers can stimulate the production of collagen, a protein used to repair and replace damaged tissue. It can also help the body form new capillaries, speeding up the healing process by carrying more nutrient-rich oxygen to damaged areas. While we feel infrared light as "heat" on the skin, most infrared lasers used in treatment are considered "cold": they are powerful enough to cause changes in cells, but not enough to cause tissue damage.
Any warm object gives off infrared radiation -- including the human body. Special heat-sensitive scanners can detect differences in heat, code them by color, and map them out in an image showing the hottest spots. Engineers use them to find heat leaks in buildings, doctors can find hidden tumors in the body, and biologists can locate diseased plants in a forest. Astronomers use infrared imaging to detect warm dust around new stars that are not yet "hot" enough to emit visible light.