Equalization (Spectral Processing)   

Equalization Types

The high shelf cuts or boosts the frequencies above some cutoff frequency by a user determined amount. The following diagram illustrates a high shelf with a cutoff frequency of about 8K and with -9 decibels of attenuation. The cutoff frequency describes the frequency at which 3 decibels of cut or boost is achieved.  The low shelf cuts or boosts frequencies below the cutoff frequency. The diagram below shows the response curve of a low shelf set to boost frequencies below 100 Hertz by 9 dB. 
The low pass filter is a special case of a high shelf cut where there is maximum attenuation of the high frequencies above the cutoff point. It passes only the lower frequencies The high pass filter is the opposite of the low pass. A low shelf cut with maximum attenuation at the cutoff frequency it passes only high frequencies
Band Pass (band limiting) 
Band pass boosting increases the level of frequencies around some centre frequency over a range determined by the bandwidth of the filter. The following diagram shows the response of a filter with a relatively narrow bandwidth 9 dB boost around the centre frequency.  The diagram below shows a wide bandwidth bandpass cut of 6 dB cut around the centre frequency. 
Graphic Eq consists of a series of narrow bandwidth filters which can be set to either cut or boost. 
1/3 octave 1/2 octave  parametric
cut, boost, sweep/frequency, Q (bandwidth) 
All EQ introduces some phase delay into a signal and when used excessively (or repeatedly) can lead to unwanted phase distortion of the resulting mix.  Use EQ judiciously, good microphone technique can help avoid the use of too much equalization. 
Be aware that EQ affects signal level, a boost in some frequency area has the potential to raise the overall level to distortion or clipping levels while cuts can reduce the impact of a sound, so some level adjustments may need to occur.  Sometimes its better to cut the frequencies around the desired range rather than boosting them directly to avoid distortion or a potentially noisy sound.