Equalisation is used for a variety of reasons for correcting, enhancing, contextualising or blending the sound source. As with all effects and processes; less-is-more and you can quickly add too much making your recordings sound worse!
The Parametric Equaliser can control three aspects of each frequency: level (boost or cut), the centre (primary frequency) and the bandwidth (range of each frequency). Each selected frequency can be increased or decreased. A parametric equalizer can also control the centre frequency. Each model varies, but the centre frequency can be varied to adjust a specific frequency. A parametric equalizer also controls the bandwidth, Q, of each control. For example, if the centre frequency is 30Hz, the control also affects frequencies as low as 20Hz and as high as 40Hz, although the affect is less at the extremes.
A Graphic Equaliser is the simplest type of equaliser and consists of multiple sliders or controls for boosting or cutting bands or frequencies of sound. For example, a typical five-band graphic equalizer has sliders for five fixed frequencies; A ten-band equalizer has controls for ten fixed frequencies, allowing greater tone control. More bands = more control.