The wavelength of light that comes out of an X-ray laser is in the tens of nanometers, versus in the hundreds of nanometers that comes out of a "regular" (i.e. visible wavelength) laser and versus the hundreds of millions of nanometers that came out of their fore bearer the maser (Microwave-aser).
But, at the end of the day, it's all light and all the same basic mechanism.
To perhaps add a few more details to the main difference mentioned already (energy of the light being produced) *and explain differences in the process of creating x-ray beams:
It might first be useful to point out that laser light, unlike that from a light bulb, is coherent, so all the light waves are in phase with one another which is a direct result of how the energy is produced. In standard lasers, this energy is coming from the relaxation of excited electrons (excited by some mechanism, pretty much always deliberate to ensure the highest probability for the relaxation of the energy you're wanting, using something like a flash lamp, electric current, or even another laser, depending on the specific type of laser). The relaxation gives off the light you're looking for. Once this happens in one electron, that light prompts a neighboring, similarly excited electron to do the same with the nice bonus of doing so perfectly in phase with the original stimulating light/photon (this is the "stimulation" and "amplification," since you're getting two for the price of one, in the acronym that makes the word laser -- Light Amplification by the Stimulated Emission of Radiation). Since this means you need to have the electrons excited to an energy at least as high as the energy you want to get out of the laser, and for a relatively long amount of time as far as atomic/molecular excitations are concerned, the higher the lasing energy, the harder the process is to control and produce.
When you reach the energy of x-rays, this is so much that it will usually just ionize whatever is being excited, stripping off the electron completely, which kills your shot at the whole coherent light production process. The first x-ray lasers got around this by using just the high energy electrons themselves, produced in [linear accelerators](https://portal.slac.stanford.edu/sites/lcls_public/aboutlcls/Pages/About-LCLS.aspx
), using magnetic field to giggle them back an forth fast enough to give of coherent x-rays.
More recently (and using technology I'm still trying to get my head around fully that totally blows my mind), [table-top x-ray](https://jila.colorado.edu/kmlabs/research/articles/attosecond-nonlinear-optics
) lasers are coming out that are made by -- in the absolute simplest of analogies that doesn't completely capture the process -- almost but not quite ionizing atoms by kind of slingshotting electrons around their nuclei in such a way that the angular acceleration it gains has the x-ray energy you want, which it then gives off as it relaxed back to its regular orbit around the nucleus. Again, the details of this one I'm still catching up on so I hopefully I'm not too off on these points and it still makes some kind of sense.
Not sure how much you already knew about laser physics, so sorry if this is overly in-depth and telling you stuff you already knew. I didn't mention some of the other aspects that will affect the type of light you get out (such as pulsed lasers vs. CW that are a single wavelength emitted constantly), but I'd be happy to further elaborate if you're interested.
The frequency is higher i.e. the energy in an X-ray laser is higher (E=hv, where E is the energy of the photon, h is the plank constant and v is the frequency), therefore more dangerous than a regular (visible-light) laser.
Awesome, thanks! Is the difference down to the geometry of the magnets? Or is there other differences between the two? (I.e. input beam properties)
I'd imagine for the broadband source you'd need an electron pulse with a larger range of kinetic energies, whilst the converse would be true for the narrow band output? Which leads me to think it's more than the geometry of the magnets....