The electromagnetic spectrum is the range of all possible frequencies of electomagnetic radiation from the low frequencies used for radio communication to gamma rays used in PET scans. Visible light is a small portion of that range (380 nm - 760 nm) that we can see with our eyes. Other species, like bees, can see outside the visual spectrum.
Sunlight is a continuous spectrum of all the visible wavelengths plus some ultraviolet and a lot of infrared. 44% of the sunlight's energy strikes the earth as visible light. At sunrise and sunset the path through the atmosphere is longer and the shorter wavelengths of blue and green are removed from the spectrum as they are scattered by air molecules. This is why the light is red and orange at this time of day.
Different light sources emit different mixtures of wavelengths: incandescent lamps produce a continuous spectrum that mimics sunlight where other types of bulbs emit only a few discrete wavelengths.
Shorter blue wavelengths of light stimulate the brain to stop producing melatonin. Devices which emit these wavelengths may alter the circadian cycle if used at night. Explore the spectrum of different commonly used light sources by clicking on the chart.