I will be talking about the kinds of dynamic processors and what they do.
Dynamic processors change the sound of audio signals by changing the dynamic range. Some dynamic processors expand the dynamic range and some compress it.
A compressor (or downward compressor) controls the volume level of an audio signal so as not to damage the speakers. To apply a compressor you must first set a threshold. The threshold is the level at which the compressor starts working. Any frequency over the threshold will, essentially, be turned down. How much it’s turned down depends on the ratio. For example, with a ratio of 2:1 all frequencies will be cut in half. With a ratio of 1:1 nothing will happen and with a ratio higher than 10:1 the compressor will become a limiter.
A limiter functions much like a compressor. The only difference being that with a limiter everything above the threshold will be stopped instead of just turned down. They are sometimes referred to as brick wall limiters.
An expander is basically the opposite of a compressor. Expanders increase the dynamic range of an audio signal by reducing the gain of the signal below the threshold. Again, how much it is reduced depends on the ratio. For example, with a ratio of 2:1 the gain in the audio signal would be reduced twice as much as it already was.
Gates (or noise gates) reduce the amount of unwanted noise, such as the hum of an amp, in an audio signal. To do this you must set a threshold above the sounds you want to eliminate but below the sounds you want to hear. How well the gate works depends on the range, attack and release. The range determines which frequencies will be blocked and which frequencies will be let through. The attack determines how quickly the gate will open and the release determines how quickly it will close. If the attack is set too low the gate will open too slowly, cutting off the beginning of the wanted sounds. If it is set too high it will open too quickly, letting in unwanted sounds. If the release is too short it can cause chattering. If it is too long it will close too slowly, letting in unwanted sounds.
To summarize: A compressor reduces the gain of signals above the threshold, essentially turning them down.
A limiter stops all signals above the threshold.
An expander reduces the gain of signals below the threshold.
A gate reduces the amount of unwanted noises by stopping all signals below the threshold.