The electrons interacting with electromagnetic waves are investigated in vacuum electronics. It needs the knowledge of plasma physics as well as the nonlinear feedback between the electron beam and electromagnetic waves. Complete Maxwell's equations of the electrodynamics should be considered. From the practical perspective, the major application of microwave tubes is to generate wave energy from the kinetic energy of the electrons, i.e. to produce radiation sources. The frequencies of the radiation sources generated by microwave tubes range from a few MHz to low THz. Different frequency band associates with different applications. This paper discusses one of an important microwave tube-gyrotron. Its working principle and characteristics will be elucidated. The key components of gyrotron including electron gun, vacuum window, and brazing of the tube will be reported in details.