Raman spectroscopy is a light scattering technique, and can be thought of in its simplest form as a process where a photon of light interacts with a sample to produce scattered radiation with different wavelengths compared to the induced light.

Raman spectroscopy is extremely information rich, (useful for chemical identification and concentration, characterization of molecular structures, effects of bonding, environment and stress on a sample).

Raman spectroscopy works by the scattering of light – 99,999% is Rayleigh scattered – 0,001% is Raman scattered light.

Wavelengths close to the laser line, due to elastic Rayleigh scattering, are filtered out while the rest of the collected light is dispersed onto a detector.

Each molecule has its own unique Raman lines – which is very useful for identification and quantification

More information available at wikipedia.