Polarization spectroscopy comprises a set of spectroscopic techniques based on polarization properties of light (not necessarily visible one; UV, X-ray, infrared, or in any other frequency range of the electromagnetic radiation). By analyzing the polarization properties of light, decisions can be made about the media that emitted the light (or the media the light passes/scatters through). Alternatively, a source of polarized light may be used to probe a media; in this case, the changes in the light polarization (comparing to the incidental one) allow to infer the media properties.
In general, any kind of anisotropy in the media results in some sort of change in polarization. Such an anisotropy can be either inherent to the media (e.g., in the case of a crystal substance), or imposed externally (e.g., in the presence of magnetic field in plasma).