From a 1994 article for the University of New Mexico’s Intro to Electronic Music class:
Equalization is a process of enhancement of audio signal. It is one type of signal processor, because it processes an audio signal.
Equalization (EQ for short) does basically one of two things: Either it boosts or cuts a given frequency or range of frequencies by a chosen level.
Outboard EQ processors come in two basic flavors: Parametric and Graphic.
Parametric EQ uses a system that allows a user to select which frequencies they’ll be working with. Thus, it is very flexible. Fully Parametric EQ has completely selectable frequencies and bandwidth. Some EQ’s are semi-parametric, meaning they have some, but not complete, flexibility. Most mixing boards use some sort of semi-parametric EQ (although many use a system that is not parametric at all, but cannot be called graphic).
Graphic EQ has no method for changing any of the frequencies that are effected, but still can be quite flexible, if it has a wide variety of bands. Graphic EQ derives its name from the fact that its sliders are configured in such a way that when a user look at them, they graphically represent the configuration being used.
Mixing boards use an EQ system that is not usually truely parametric, but it is not a graphic system (since, of course, it does not use sliders that graphically show the configuration). Many boards use a fixed band EQ in which each knob effects multiple frequencies, such as high, mid and low (unlike graphic EQ in which each slider boosts or cuts only one frequency). Many boards use a semi-parametric system in which some sections of the EQ have a knob which will choose a frequency, which is then usually accompanied by a bandwidth selector, which chooses how wide an area of frequencies will be effected. (This is nearly true parametric EQ, but usually only one or two frequency ranges provide this option, so we don’t normally call this a truely parametric EQ system.) Some boards have sections of their EQ that are semi-parametric, because the frequency effected can be selected, but not the bandwidth.
EQ is used for many purposes, including enhancing an audio track, making a track more audible in a mix, fixing noise problems, helping audio tracks to sound more real or artistic effects such as “telephoning” (my word, meaning to cut low and some high frequencies, making a track sound as if from a telephone).