Light is measured by its wavelength which is measured in Nanometers (nm) or frequency which is measured in Hertz.
The wavelength of light determines its characteristics. For example short wavelengths are high energy gamma-rays and x-rays. The whole range of wavelengths is called the Electromagnetic Spectrum. The wavelength equals the distance between two wave crests. See below the whole spectrum and also the visible light spectrum.
Frequency is how often the waves pass over a period of time, normally one second. A low frequency or Hertz in a light can cause flickering. This is most common on fluorescents with a frequency less that 50hz. Although we can't see most fluorescents flicker because they are higher than 50hz some individuals sensory systems can detect the flicker and be affected by it. Since the introduction of fluorescents in the workplace there have been complaints of headaches and eye strains.