Did you know that light has temperature? Most of us have noticed that light seems to have different colors based on different times of day, weather conditions, and if it is natural or artificial light. Well, those color differences are measured by temperature. The Kelvin scale measures the amount of heat reflected by different light sources under controlled conditions. Because real-world lighting is not under controlled conditions, the actual color temperature of light varies widely.
The Kelvin scale generally used with photography ranges from red to white and then through blue. This covers a visible light spectrum and not the entire light spectrum. An easy way to remember the progression of light colors is the acronym "Roy G Biv". This stands for "red, orange, yellow, green, blue, indigo, violet." This does not match exactly the way light colors behave in photography but it is a good place to start. Green tends to show up only occasionally in lighting situations with "white light" occupying green's place most of the time.
Contrary to what colors we tend to think of as "hot" and "cold", red is actually a cooler temperature than blue according to Kelvin scale. Still, in photographic terms, we refer to bluish light tint photographs as "cool" and reddish light tint photographs as "warm".
Color temperature is often used in digital darkroom software for making color corrections to images. It is also used to know what type of tint particular lighting will create in images. The most common usage of the Kelvin scale in photography is to determine white balance.