The possible polarisations of light are due to the fact that light (and, indeed, any electromagnetic wave) propagates as a transverse wave in the electric and magnetic fields.
In an isotropic medium (including in a vacuum) a light wave can be expressed as the superposition of two plane waves which are perpendicular to one another. The waves have the same amplitude, but one is phase shifted with respect to the other.
With a general phase difference, this produces elliptical polarisation. If the phase difference is $\pm90^o$ then this produces circular polarisation in one direction or the other. If the phase difference is either zero or $180^o$ then this produce linear polarisation. Both circular and linear polarisations are special cases of elliptical polarisation.
To specify the polarisation of a light wave (or any transverse wave) you therefore need two parameters. One defines the angle of the plane wave components (relative to a vertical or a horizontal plane, for example) and a second defines the phase difference between the plane wave components.
Note that this behaviour is classical behaviour that is common to all transverse waves. The quantum mechanical nature of light becomes apparent when we consider the polarisation states of individual photons.