  This report is a primer introduction on the sources of coherent syncrotron
radiation from FELs compared to conventional lasers. Even though the underlying
mechanisms are different, FELs can be considered a laser source to all effects.
In fact FEL is based on the stimulated emission of virtual photons by a
relativistic electron beam, passing through an undulator/wiggler, much like
conventional lasers exploit the stimulated emission by an atomic/molecular
system on which a population inversion has been realized. FEL operation is
illustrated by means of the usual set of parameters (gain, saturation
intensity...) used to describe the functioning of conventional lasers.
