Gamma is a non-linear operation used to code and decode brightness values in still and moving imagery.
The term gamma is applicable in photographic terms when we want to view our images on computer monitors. For each pixel in a digital image, you'll find a certain level of brightness, which is given a value. The computer monitor uses these values when displaying the digital images. However, CRT and LCD computer monitors must make use of these values in a non-linear way, meaning the values must be adjusted before they are displayed.
Straight out of the box, a computer monitor usually has a gamma of 2.5. However, most modern DSLRs shoot with a color space of either sRGB or Adobe RGB. These work on a gamma of 2.2.
If a computer screen isn't calibrated to match this gamma of 2.2, images from a DSLR can look too dark and completely unlike the images shot in the first place! Hence, it is important to understand the effects of a gamma setting on both camera and computer.
You can use a variety of methods for calibrating your monitor, including hardware and software options. Visit the Desktop Publishing page on About.com for more information on how to calibrate your screen.