Compression refers to one of the mostly discussed spheres of dynamic processing of a sound. But there are many other possesses except it for working with the dynamics of a mix, which are worth paying attention to as well.
An expander is opposite to a compressor. While a compressor reduces the volume level of signals that exceeds the threshold, an expander decreases the volume level of signal that is lower than the threshold.
It can be used at online mixing service to clear acoustic sounding of instruments from extra side-tones of a room, emphasizing of drum, piano or bass attacks. As an alternative it can be used for super rounding-off of drum sounds if you adjust the threshold very low and with very slow time of an attack.
2. A Gate
Gates are an extreme version of an expander. They clear all sounds which are below the threshold. They are ideal for creating stutter effects, working with drums and pads, controlling the tail of reverberation or deleting unnecessary noise, for instance, at backing vocals.
3. Expanders increasing the volume level
This kind of processors work on a principle of adding loudness to signals which exceed a threshold making loud sounds even more louder and not the reverse, the way it do ordinary expanders. They are perfect for adding an attack or an exsiter effect for such highly dynamic sources as vocal, piano or an acoustic guitar.
4. Modulators of transients
It is a combination of an expander and compressor, reacting exclusively on very quick parts of a signal. They can be used for decompression poor hip-hop bits, adding light saturation to instrumentals, increasing signal attacks with long sustain (the stringed instruments e.g,).
5. A Limiter
A limiter is like a super compressor. They are intended for working with a short attack and use very high values of ratio. They are appropriate for placing vocals inside the mix, cutting excess transients and for increasing the loudness of an output signal.
All of these types of processors are what I use creating my before after music mixing and mastering demos.