Gamma refers to the relationship between the voltage supplied to your monitor and the intensity of the display. Rather, gamma correction lets you extract the largest amount of useful visual information from a small amount of data. Before I go any further, I'll have to define a few terms.
Intensity measures the amount of light coming from a surface.
Unlike some of the other terms, intensity can be measured accurately, without having to be adjusted for the subjective approximations of visual perception. Brightness, according to the CIE, refers to how much light an area appears to emit. Since brightness is subjective rather than objective, luminance is often used as an equivalent term. Luminance is defined as radiant power weighted by a set of defined visual characteristics.