A parametric EQ permits the operator to not only set the cut/boost of a given band, but also the center frequency and the BANDWIDTH of that band. Typically, a band may be as narrow as, say, 1/6-octave, to as wide as 3.0-octave. The center frequency of the band is tuneable over a wide range, as well. So, instead of a single control per band, there are 3. A "semi-parametric" EQ has cut/boost, a frequency "sweep" control, but no bandwidth control.

Parametric EQ's excel at "honing in" on a problem frequency band and minimizing it. This can be a loud "hot spot" on a bass guitar, or a resonant node prone to feedback in a live situation. You can also set up a much wider band than the typical graphic EQ can do, allowing you to use it as a gentle "tone control".