This effect was discovered by sir C.V. Raman; hence the name.
The elastic scattering, the commonly observed and most of the photons undergo this, is also called Rayleigh scattering. In this the scattered photons have the same energy (frequency and wavelength) as the incident photons. But a small fraction of the scattered photons have a different wavelength and frequency. Normally such photons have less frequency (greater wavelength) than the incident photons.