It's a clipper, I really don't know what's the difference between a clipper and a limiter, this clipper reacts like a limiter to me.
A limiter has attack and release behaviour - sample levels exceeding the threshold will be passed, being GRADUALLY attenuated to the threshold level while the attack time elapses, and subsequent levels below the threshold will continue to be reduced by a graduallt decreasing amount until the release time has elapsed.
A clipper by contrast is instantaneous - clippers are a "waveshaper" type plugin, meaning they map in input sample value to an output sample value on a per-sample basis. Any waveshaper plug-in can be set to a clipper transfer curve. Dedicated clipper plug-ins are waveshapers which can only be set to a clipper-type transfer function.
So with a hard-knee clipper the effect is: any sample equal or below the threshold remains unchanged, and any sample above the threshold is re-valued to the threshold value. The peaks of the waveform are flatted. Clipped. Clipper. See?
The knee (or "saturation") parameter of a clipper allows samples approaching the threshold level to be attenuated by gradually-increasing amounts.
If this sounds a lot like overdriven distortion, that's because that's what it is.
And, like distortions, some clippers may do other stuff (pre and post-eq, etc) to modify the sound, so two clippers displaying the same settings might not sound alike.
A free VST waveshaper plug called Cyanide2 is available
http://www.smartelectronix.com/~bram/ which displays the transfer curve and also the output you would get if a pure sinewave was input, which gives you some idea of what goes on. The transfer curve can be edited to a clipping function, or to a whole load of more complex transfer functions which are probably not very useful in the average metal mix!
If a plug-in has a release and/or attack time setting it is a limiter, not a clipper.
I get confused too - in another post I referenced Sweetboy's Volcano plug while discussing clippers, but firing it up the other day I noticed it had a release control, so it must be a limiter. D'oh!
BTW if a limiter allows attack and release settings of ZERO it will act exactly like a hard-knee clipper. This is avoided beacuase limiters (and compressors) are disigend to avoid distortion of the waveform, whereas clippers are designed to create it.