If i recall right, "Q" is the quality factor of a filter related to the frequency it controls and the bandwidth over which the control is made. Mathematically it is a unitless figure expressed as the ratio between the center frequency in Hz and the corresponding bandwidth also in Hz. High Qs mean narrow bandwidth. Low Qs, wide.
An equalizer is essentally a device with multiple RLC(resistor, inductor, capacitor) filters at certain points on the audible spectra usually in increments of 1 octave between center frequencies up to 1/3 octaves. A graphic equalizer has fixed center frequencies and Qs and its name derives from the fact that its slider positions makes a graphic statement of the relative strengths of the frequencies it controls. They have fixed positions for center frequency, bandwidth and amplitude levels. (though there are graphics that have selectable +/- 6db or 12db level controls)
Parametric equalizers are more complex and thus more expensive. The name derives from the fact that the frequencies can be controlled using parameters such as amplitude, bandwidth or Q and center frequency that are supposed to be independently variable. (Some so called parametrics have such variable parameters but are not independently controllable.) You basically have 3 controls per frequency band. And many parametrics only sport fewer than 10 possible center frequencies (your choice). Most if not all don't use sliders but pan pots and you would have some difficulty checking the relative gains and dips of the various frequencies, unlike in graphic eq where the sliders position give you an idea.
The best equalizers are those using 31-band 1/3 octave sliders as you get more control over the audible spectra. Even better if they have narrow Qs. Having a narrow Q means the center frequency gets boosted or attenuated with the least effect on the adjacent frequencies before and after it. A low Q means a wide bandwidth where a wider swath of frequencies before and after the center frequency gets boosted or attenuated together with the center frequency in a bell or inverted bell shaped frequency curve.
Paramatrics are prefered because you have better control on each frequency you choose to cut or boost. But the trade-off is fewer frequencies to control and unverifiable response curve. But a good graphic equalizer with narrow Qs can do a good job as well.
