Infrared (IR) radiation is a specific form of electromagnetic radiation, which is a form of energy that travels as a wave with a given frequency (number of waves in a certain time) and wavelength (distance between two wave-tops) that can be absorbed or emitted by matter.
When radiation has a high frequency, it will have a shorter wavelength (to "fit" more waves into a time period). Visible light, which constitutes the colors seen in day-to-day life, is a higher frequency form of electromagnetic radiation. The sun emits mostly visible light, while Earth emits and absorbs mostly infrared radiation, also commonly referred to as "thermal radiation."
Earth emits and absorbs mostly infrared radiation