Steam hammer, AKA steam flashing, flow shock, differential shock, or the rather clumsy 'water hammer in steam systems', is a variant on water hammer that occurs in systems that carry both steam and water. Because the difference between steam and water is primarily temperature, many systems that should ideally carry only steam will in fact carry some condensed water under some conditions.
Steam hammer occurs when steam condenses and a layer of water (usually referred to as 'condensate' in technical jargon) forms in the pipe. Small amounts of condensation don't usually cause a problem, and all modern steam systems will have systems in place to deal with condensation. However, if something goes wrong and water builds up, things can get suddenly explosive.
When water and steam travel down the same pipe, the steam will tend to move faster -- much faster -- than the water (same pressure, less mass; this is a condensate bi-phase system). This causes turbulence in the surface of the water. This starts out as ripples, becomes waves, and eventually may get to the point where a tall 'wave' entirely blocks the pipe; the steam pushes this slug of water down the pipe at high speed, picking up more water as it travels, until it hits an obstacle.
At this point, it is approximately the same in effect as water hammer, except that it is very hot, moving very fast, and occurring in a system not designed for water and water hammer. Steam hammer can tear welds, rupture pipes, cause valve failures, and severely damage any humans in the vicinity.
Steam hammer may also refer to bubbles of steam forming and then collapsing, especially when they collapse upon hitting the wall of a boiler, radiator, or water tank. This is noisy, but does not generally result in sudden, painful death. However, this can still be damaging in high-pressure systems, and when in doubt, check with a professional.
A steam hammer or drop hammer is also a industrial strength, extra-large piston-driven hammer, which it used for pile driving and large scale forge work. It is so called because it is powered by steam, and because it hammers stuff.