It's a question of what you mean by "light". Usual "thermal" light, say from the sun or a good old light bulb, is not polarized at all. It's an incoherent superposition of a lot of randomly polarized field modes (it's incoherent, because the relative phases of these modes are randomly distributed too).
If you have laser light, you have to a pretty good approximation, a coherent and monochromatic em. wave. Such a wave can be characterized by the electric field. Taking the [tex]z[/tex] direction of a Cartesian coordinate system as direction of wave propagation, it can be idealized as a plane-wave solution of the free Maxwell equations:
[tex]\vec{E}(t,\vec{x})=(A \vec{e}_x+B \vec{e}_y) \exp(-\mathrm{i} \omega t+\mathrm{i} \vec{k} \cdot \vec{x})+\mathrm{c.c.}[/tex]
Here, [tex]A[/tex] and [tex]B[/tex] are arbitrary complex constants, that determine the polarization state as a superposition of horizontally and vertically polarized field modes. This most general polarization state is called elliptically polarized light.
Special cases are
linearly polarized light: B=r A with [tex]r \in \mathbb{R}[/tex], i.e., both parts of the wave are in phase
circularly polarized light: [tex]B=\pm \mathrm{i} A[/tex], i.e., there's a phase shift of [tex]\pm \pi/2[/tex] between the parts (called right or left circularly polarized em. wave).
