Output devices have a non-linear relationship between say light output and input voltage. This is often expressed as a power law: out = ingamma

Almost every  monitors have a intensity to voltage response curve which is roughly a 2.5 power function. This just means that if you send your monitor a voltage 0.5 hoping for half full-intensity, it will actually display a pixel at 0.5 ^ 2.5 = 0.177.

 Sample Input to Monitor  Graph of Input
 Output from Monitor      Graph of Output  L = V ^ 2.5
The eye does not see intensity/light energy linearly! It has a "gamma" style curve of around 0.5! This almost cancels out linear A/D conversion. A gamma correction to the image compensates for the monitor gamma being nearer 2.5. For printing etc the curves are much more complex but the principles are similar.

See this tutorial on gamma for more (where the above is adapted from).