Surprisingly, this might be the hardest question to answer about light. There have been many theories about what light actually is. The classical theory is the one we will explain, and it says that light is made of waves. Each wave is distinguished by 4 bais components: amplitude, frequency, wavelength, and speed. Many of these components are interrelated.

Simply stated, light is a disturbance in an electromagnetic field.  Like a disturbance on a water surface light will spread out away from the initial point of disturbance in all directions. The water does not really move on the surface, it just moves up and down at the same place creating a wave.  Electromagnetic waves are the same; when a charged particle oscillates in place the electromagnetic field around it is disturbed, and the disturbance travels like a wave away from the particle.

Amplitude 

The amplitude of a wave is the height of the wave (see below.)  Amplitude is the strength of disturbance in a way.
 

Frequency

Frequency is the number of cycles that the wave goes through per second. The period of the wave is 1/frequency or the amount of time per cycle.
 

Wavelength

Wavelength is the horizontal distance between two successive similar points on the wave, such as from crest (highest point of the wave) to crest, or trough (lowest point of the wave) to trough.

Speed

The speed of light in a vacuum is C, which is 300,000 km/s. The wavelength, frequency, and speed are all related. The frequency times the wavelength is equal to the speed, or F*W=C.


The Doppler effect:

As you may already know, the wavelengths of light that are visible to the human eye are from about 4 ten-millionths of a meter to 7 ten-millionths of a meter. Each one of these wavelengths corresponds to a different color of light, the longest wavelengths corresponding to the red side of the spectrum, and the shorter ones corresponding to the violet side of the spectrum. Just like a rainbow the colors of the spectrum go ROYGBIV or red, orange, yellow, green, blue, indigo and violet.

700-600nm	Red
600-585nm	Orange
585-565nm	Yellow
565-500nm	Green
500-490nm	Cyan
490-460nm	Blue
460-400nm	Violet

This is in order from longest wavelength to shortest. If something a constant distance away emits a constant wavelength of light, the wavelength of the light that gets to your eye will be the same as the wavelength of the light that was emitted. If the source starts moving towards you, each time the source emits the crest of the wave (or any other part of the wave) it will be nearer to you, so the crests will be closer together. This results in a shorter wavelength, which means the light you see will be closer to the violet end of the spectrum than it actually is. This is called blue-shifting. The same is true of a source moving away from you, only the wavelengths will be longer, and therefore the light will be closer to the red end of the spectrum. This phenomena is called red-shifting. Both blue-shifting and red-shifting are part of the Doppler effect.

When an electron is given energy, such as heating up a substance, it jumps up energy levels in the atom. The higher the energy level, the more unstable the atom, but the more energy the atom has. The electron has to quickly loose this extra energy in order to keep the atom stable. As the electron drops down energy levels it emits the energy off as light. The amount of energy emitted corresponds to the wavelength of light emitted. The more energy given off, the lower the wavelength of light. This is because higher energy waves have shorter wavelengths. The wavelength of the light is not only determined by the energy put into the electron in the beginning, but it is also determined by the element of the atom. Every atom has its own emisison spectrum, which means every atom has only certain colors of light it can emit.