Raman spectroscopy is a rare experimental technique in surface science, but it is major in chemistry. Recently it becomes popular in Graphene research and strained semiconductor research.
In simple word, Raman spectroscopy is "light version of EELS (electron energy loss spectroscopy)".
It is "light-in light-out" technique. In this experiment, we measure energy spectrum (Intensity-energy curve) of the out-going light.
Most of the incident light is scattered without loosing energy (elastic scattering).
It is often called Rayleigh scattered light.
A small portion of the incident light gives its energy to some phonon modes and thus loose energies of the corresponding phonons.
The light with a smaller energy appears as a peak in the out-going light spectrum.
We detect the inelastic peaks in the energy spectrum, and determine the phonon energy of the sample.