The gradient determines the color of the fire.
U Frequency and V Frequency determine the fineness of the structure. Thus U=1 and V=1 results in regular flames, U=1 and V=0.25 gives rather elongated flames.
T Frequency affects the speed of the flicker, i.e., how quickly the flames change — the higher the value, the faster the flicker.
Turbulence determines how violently the flames break up in a notional wind. A setting of 2 doubles the wind; a setting of 0 suppresses all wind.