Doppler radar uses the Doppler effect to measure the speed (radial velocity) of targets. Radial velocity is how fast the target is coming towards, or going away from, the radar. The Doppler effect will shift the received frequency up or down, based on the radial velocity of a target in the beam. This gives a direct and highly accurate measurement of target velocity, but only the radial velocity. Doppler radar by itself doesn't measure the azimuthal velocity (how fast it is going in other directions).
Doppler radars may be Coherent Pulsed, Continuous Wave, or Frequency Modulated.