A maser is a device that produces coherent electromagnetic waves through amplification by stimulated emission. The word MASER stands for Microwave Amplification by Stimulated Emission of Radiation.
When the electrons are emitted from the lower energy band towards the higher energy band by pump input, the population inversion takes place at the higher energy band. Now the electrons fall back towards the lower energy band. At this time they emit the photon's which generates the microwave frequency. The generation of frequency produced by the photons depends upon the gape between the energy bands. In case of maser this gape is created by different methods for example the magnetic flux in case of Ruby Maser.
There are different types of MASER designed for amplification and oscillation at microwave frequencies in which the following types are widely used.