Equalizers are used to set the balance between different frequencies. The signal as it comes out of the element in a guitar, will sound beter in most cases with the right use of an equalizer (EQ). A lot of guitars with an element and preamp have a two- (bass and treble) or threeway (bass, middle and treble) equalizer. This comes in useful to accentuate a certain frequency range, but when one has some more detailed wishes it is advisable to use a more extended equalizer.

Because every room has it's own acoustic characteristics, sound will never sound the same. Some rooms for example dampen the higer tones more than the lower ones. To compensate for this loss of the high frequencies an equalizer can be used.

Besides that, an equalizer is a useful tool to eliminate the sound you hear when something touches the guitar. Especially when you play on bigger stages, a subtle knock on the soundboard is enough to cause the whole building to shake. With an equalizer you can easily turn away low frequencies, so you do not need to practice don't touching the wood.

Graphic equalizer
A graphic equalizer is easy to operate. The overall frequency spectrum is divided into several segments and of each segment the volume can be adjusted separately. This is the same type of equalizer that is build in in most guitars which have one. Professional equalizers sometimes do have over thirty segments. For an acoustic guitar are seven segments in most cases sufficient. On the left you can see a Boss GE-7.
Parametric equalizer
The image on the right show an AER Dual Para EQ. A parametric equalizer is used to strengthen or weaken specific frequencies. In general, there are three important knobs:

Frequency: At what frequency changes take place.
Bandwidth: Adjusts the sharpness of the peak, the surrounding frequencies are pulled so one can create a smooth gradient.
Level: The volume change around the set frequency.

If one wants to make changes on several frequencies, more equalizers are needed. De illustrated parametric EQ has two channels.