Polarization refers to the direction with which the electric field of light waves oscillate, which is perpendicular to the direction of propagation. Light waves can be linearly, circularly, elliptically, or randomly polarized. For more information about polarization read Introduction to Polarization.
Laser sources may be polarized due to anisotropy (a material property that is different in different directions) in the laser gain material, directionally dependent polarization losses in the laser resonator, or the use of birefringent optical materials. Some laser sources are unpolarized (e.g. fiber lasers). The polarization state of a laser can also be used to reduce unwanted and potentially dangerous reflection from high-power sources as some materials reflect or absorb light in certain polarizations states over others.
Many laser applications including some interferometry, optical amplification and modulation, nonlinear frequency conversion, and incoherent and coherent polarization beam combining (polarization coupling), depend on the state of polarization in order to function.