Within Limits
We are often told that a limiter is simply a compressor with an infinitely high ratio, so that once a signal reaches the threshold, it is prevented from exceeding it. This is pretty much true, but in this digital age where even very short periods of clipping may not be acceptable, a regular compressor is unlikely to be able to act fast enough to function as an effective limiter — fast transients can pass through a system before your compressor is able to react, and this can result in clipping at your A-D conversion stage. In the days of analogue tape, this didn't matter so much, as short periods of analogue overload tended to be inaudible, but some digital systems can't cope with any clipping at all, however brief. In such situations, a dedicated, fast-acting limiter is the best bet.
In order to control signal peaks without affecting a sound's subjective level, some digital limiters may be programmed to allow a certain number of samples to clip before the level is reduced. In situations where the equipment next in line doesn't object to short periods of clipping, this can actually make the material seem much louder, though, as a rule of thumb, the period of clipping should be less than 1mS, which is equivalent to 44 consecutive samples at the sampling rate of CD-quality audio. However, if frequent clipping is expected, then the maximum length of clipped signal should be reduced to below 10 samples, as research indicates that repeated clipping within a short space of time is more audible than widely spaced instances of clipping. Some limiters emulate analogue soft clipping, where the top few dBs of any peaks are rounded off rather than clipped. Soft clipping can also help preserve the impression of loudness, though the effect can be audibly unpleasant if the signal is forced into limiting for more than very brief periods of time.