If we approximate the potential energy curve of a vibrating bond as a parabola (i.e. assume that the vibration is harmonic) then the specific selection rule for vibrational Raman transitions is Δν = ±1.
Transitions for which Δν = -1 lie to high frequency of the incident radiation, and are called the Stokes lines.
Transitions for which Δν = +1 lie to low frequency of the incident radiation, and are called the anti-Stokes lines. These are usually more intense than the Stokes lines, as the Stokes lines require the molecules to initially be in excited vibrational states, which is a relatively rare occurrence.
