A polarized EM wave is an EM wave that has a preferred direction for the electric and magnetic field vectors in the wave. In unpolarized light, there is no preferred direction: the electric and magnetic field vectors are distributed uniformly in the plane perpendicular to the way the wave is moving.

In linearly polarized (or plane polarized) light, the electric field vectors all lie in one plane. So do the magnetic field vectors, because they're perpendicular to the electric field vectors. Most light sources emit unpolarized light, but there are several ways light can be polarized. These include reflection, selective absorption, and scattering.