Volume is the most basic form of gain control.This is the effect found on practically every electronic device that ever made a sound. This is what makes a signal louder or softer.
Compression is a more advanced form of volume control. Compressing the dynamic range of a signal makes the loud parts softer, and the soft parts louder. This is great if you want to keep the volume loud enough to hear, but prevent it from getting too loud. This can be a great help when trying to control the volume of a live performance, or mix a prominent track in a recording.
Expansion is the opposite of compression. Expanding the dynamic range makes the loud parts louder, and the soft parts softer. This can add life to an otherwise dull signal. Limiting is similar to compression in that it makes loud parts softer, but the softer parts are left unaltered, allowing for more dynamic contrast without getting too loud.
Gating will completely cut off any signal that goes below a certain volume. This can stop the sound of background noise during the silent parts of a recording. Overdrive/Distortion are mostly used for electric guitar signals. They create the “dirty” sound of an overdriven speaker. This is you basic “rock and roll” sound. Tremelo is a modulation in volume used to simulate the sound of a rotating speaker cabinet.
Equalizing is another advanced form of volume control. Instead of controlling the volume of the entire signal at once, you can specify what frequencies will be made louder or softer. Almost every home stereo system has some form of an equalizer (or EQ). This is used to change the tone of a signal to make it lower or higher sounding. Tone control is not to be confused with pitch control, which will be discussed later.
These effects can be used individually, or several can be used together. Using the effects in different orders will also change the overall sound. Figuring out when to use what effects can take some experimenting, but it is a skill worth learning if you want to work with any type of amplified signal. So next time you turn on a listen to a recording, think about what might be going on with the signal on the other side.