The main peculiarities of a Free Electron Laser (FEL)  are directly related to the physical process with which the coherent radiation is produced. In all free electron devices, the active medium is no more represented by an atomic or molecular system, but by an electron beam (e-beam) which is "pumped" by a structure (a magnetic undulator in the FEL case) which provides synchronism between the e-beam oscillations and the phase velocity of the electromagnetic (e.m.) wave generated in the same structure. The output frequency is therefore not related to fixed quantum level transitions, but to the e-beam energy and to the undulator geometry, and therefore can be quite easily changed making the FEL a tunable source. Also the saturated output peak and average power can in principle be very high and not limited by the dissipation capability of an active medium like in conventional lasers.
A. Doria, A. Doria,
"FIR and MM compact FEL", Proc. SPIE 1576, 16th International Conference on Infrared and Millimeter Waves, 15767Q (4 December 2017); doi: 10.1117/12.2297991; https://doi.org/10.1117/12.2297991