infrared

To be a bit more clear, if you take the spectrum of visible light, and look down towards the end where it gets red, just below that is the infrared spectrum. Quite literally, "under red". If you go farther, you go towards the realm of radio waves.

Of course if you go the other way, "over violet", you start into the ultraviolet spectrum. Higher than that, and you are into X-rays and cosmic rays.

Infrared radiation is given off by objects as they radiate off heat. Because of this, infrared radiation is often called a heat ray. Nearly anything that is heated gives off IR. A more complete discussion can be found at black body radiation

Humans typically cannot see IR radiation. Video cameras often can see IR and translate it into the visible spectrum when shown on a television screen. This feature is used in night vision technology, where you can either illuminate something with IR, hence be able to see in the dark, or view the IR coming from a heated object -- like a human body hiding in the bushes.

Bulk IR is used for heating in the form of a heat lamp. The lamps over the french fries at McDonald's give off much more IR than normal spotlights, and so keep the fries hot.

Lower-wattage IR, in the form that is generated by LEDs, is used as a signalling medium for computers (IrDA) and in almost all remote controls for consumer electronics.

Some lasers operate in the infrared spectrum. These are especially dangerous to use, since you cannot see the beam and your eyes have no heat receptors. You are in constant danger of cooking your eyes.