What is CD ?

To understand what CD actually is, one has first to know what is a circular polarized beam of light.
To do so, let us first consider a plane polarized light.
As you may know, like any other electromagnetic radiation the light has a magnetic and an electric component. Let us neglect the magnetic one; however, keep in mind  that what follows can be exactly applied to the magnetic component as well.
Because we are considering a plane polarized light, we see an oscillation of the electric field vector intensity on a plane that has the same direction of the beam (red light). If we choose a point in the beam direction, we see that the intensity of the electric field vector (magenta) vary from a positive maximum (t1) to a negative minimum (t3) during time.

Now here is the trick.
By means of the usual vector decomposition rules,  we can decompose the magenta vector in two green vectors of equal intensity.
The change in intensity of the magenta electric field vector causes these two green vectors to rotate oppositely, each of them describing a circumpherence (see the picture).
A beam of polarized light is described by the set of each of the two green vectors along the original plane polarized light direction. As a consequence, we can also say that a plane polarized light can be seen as resulting from a right (clockwise) and left (anticlockwise) circularly polarized beams of light.