Laser means "Light Amplified by Stimulated Emission of Radiation". The Light emitted is coherent because the radiations emitted from different atoms have definite phase relationship with each other. This light has a very high degree of coherence and is almost perfectly parallel.
Emission of light from an atom of an element consists in transition of the electrons in the atom from higher (initially excited) levels to lower energy levels.  But as this is a random process, there is, in general no phase relation between different transitions and many transitions are possible giving, in general, many spectral lines. If it is possible to confine the transition to a pair of energy levels, greater amount of energy is present in such radiation. Again if all the transitions take place in phase, the radiations will be directed and enormously intense. Such action is achieved in laser.
Thermodynamically radiation occurs from excited atom in two ways. The natural type of emission in random directions with random phase relations in called spontaneous emission. It occurs in a time interval of about 10nano-seconds after the excitation. Stimulated emission is caused by the presence of a radiation (wave) of suitable frequency reacting with the excited atom. The important features of this latter type are all the radiations are exactly similar to the inducing radiation, have same phase, same polarization state, same frequency and same direction.