Infrared waves, like visible light, exist on a spectrum. The waves at the far end of the infrared spectrum (known as far infrared waves) are felt as heat. Any object emitting heat is emitting infrared waves. Thermal lights used in fast food restaurants, for example, keep food warm by bombarding the food with infrared waves.
Infrared transmitters and receivers are commonly used in remote controls for television sets, DVD or VCR players and other items. These remotes use a pulsing infrared beam, generated by the remote control, that is received by a component in the television or DVD player. Most remote controls emit infrared waves at around 36 kHz, but anything in the 30 to 60 kHz range is usable for this application.
The earth and the earth's atmosphere absorb infrared radiation emitted by the sun and other astral bodies. The earth also emits its own infrared radiation. At the same time, it reflects some of the sun's infrared radiation back into space. Satellites can use thermal imaging equipment to record and measure how much infrared radiation the earth is emitting or reflecting at any given time. Thermal imaging allows scientists to study subtle variations in temperature across the earth's surface and in clouds in the earth's atmosphere.
The nuclear and gravitational processes that facilitated the birth of the universe via the Big Bang released a lot of energy in both heat and light. This energy has, over billions of years, been absorbed by intergalactic dust and other astral bodies. The dust and astral bodies now emit some of that energy in infrared radiation. There is so much of this "background" infrared radiation in the universe that scientists can study disruptions in the background radiation to detect and measure planets, stars or other bodies they can not see otherwise.