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Equalizers are...harmonic and partial filters. Their specificity lies in the fact that they not only can get rid of component frequencies, but that they can also amplify chosen frequency zones. Of course, if there isn't anything in the signal in that range, only hiss will be added! Good EQ sections generally have 4 bands. Each offers at least 2 controls: frequency adjustment and gain. These are called semi-parametric. There's often a third setting called the bandwidth or "Q" which has the purpose of  enlarging or  tightening the frequency range (bandwidth) of the filter. When this 3rd control is present, the Equalizer is then called a parametric equalizer. Frequency adjustment will be tweakable between the upper and  lower limits of the sub-range of the filter (with software these limits no longer exist!)

The  gain knob defines, in dB, how much the filter will effect the chosen frequency. As we can see here in fig. 8, borrowed from cubase, this gain can be positive or negative. We can also see that the curve of the bandwidth can be wider (hump shape) or narrower (peak shape). This shape corresponds to the bandwidth which is adjusted by the Q setting.