A laser beam can be used to slow atoms in an atomic vapor by virtue of the Doppler effect. Atoms can "observe" an increase in frequency of the laser light if they are moving towards the direction from which it comes. This is essentially the same effect that you can observe for ordinary sound waves (e.g. a decrease in frequency of the siren that occurs when an ambulance passes by).
The slowing comes about if the laser is tuned slightly below the atomic resonance so that atoms which happen to be moving towards the beam "see" the laser as "on resonance" and begin to absorb and emit photons. Each photon absorbed or emitted gives a momentum kick of hf/c (where h is Plank constant 6.6*10^-34 Js, f is the laser light frequency, and c is the speed of light). All of the absorbed photons come from the same direction causing an average force on the atom. However, the emission photons are emitted in all directions causing the average force for this process to cancel to zero and giving an overall net force in the direction of the laser beam.
The light pressure force on the atoms can be quite large (up to 10,000 times that due to gravity), which makes it a very effective means of manipulating atoms. Since the force is in the opposite direction of the atomic motion, the atoms are subsequently slowed. A generalization of this technique to three dimensions (called optical molasses) that uses six laser beams from opposite directions has resulted in cooling an atomic vapor to within one-millionth degree of absolute zero temperature.