Laser light is effectively monochromatic. Polychromatic light is described by many different frequencies. Nearly every light source is polychromatic. Although frequently associated with light, the term can be applied to any wave phenomena. A continuous spectrum is one in which every frequency is present within some range.
Blackbody radiators emit a continuous spectrum.
A discrete spectrum is one in which only a well defined set of isolated frequencies are present. The excited electrons in a gas emit a discrete spectrum. The wavelength of a light wave is inversely proportional to its frequency. Light is often described by it's wavelength in a vacuum . Phase differences between light waves can produce visible interference effects.