There are many kinds of EQ. Each has a different pattern of attenuation or boost.
A peaking equalizer raises or lowers a range of frequencies around a central point in a bell shape. A peaking equalizer with controls to adjust the level (Gain), bandwidth (Q) and center frequency (Hz) is called a parametric equalizer. If there is no control for the bandwidth (it is fixed by the designer) then it is called a quasi-parametric or semi-parametric equalizer.
A pass filter attenuates either high or low frequencies while allowing other frequencies to pass unfiltered. A high-pass filter modifies a signal only by taking out low frequencies; a low-pass filter only modifies the audio signal by taking out high frequencies. A pass filter is described by its cut-off point and slope. The cut-off point is the frequency where high or low-frequencies will be removed. The slope, given in decibels per octave, describes a ratio of how the filter attenuates frequencies past the cut-off point (eg. 12 dB per octave).
A band-pass filter is a combination (in series) of one high-pass filter and one low-pass filter which together allow only a band of frequencies to pass, attenuating both high and low frequencies past certain cut-off points.
Shelving-type equalizers increase or attenuate the level of a wide range of frequencies by a fixed amount. A low shelf will affect low frequencies up to a certain point and then above that point will have little effect. A high shelf affects the level of high frequencies, while below a certain point, the low frequencies are unaffected.
Source: Equalization — wikipedia
See also: T1 ToneMatch® Audio Engine / zEQ