A graphic equalizer is basically a bank of filters used to boost or cut at a specific frequency. Typically the boost or cut goes between +/-12dB and with a constant Q. The number of filters depends on the application and is typically expressed as octave separation between filters. E.g. 1/3 octave graphic equalizer means the center frequency of each filter is placed at 1/3 of an octave away from the previous and next filter. 1/3 Octave filters are typically 31 bands while 2/3 octave are only 15 bands.
What are the applications? Graphic equalizers are the perfect tool to modify the frequency response of your system however they use a substantial amount of DSP power (31biquads) which is wasted most of the time. A Parametric Equalizer is more powerful for that reason.
Want to learn more, have a read at this application note.