ps sorry about my brief response - have more time now
in short
its when you take a couple of versions of the same picture (at different exposures) and stack them together
most SLRs have a built in function called bracketing which will allow you take these photos (eg one normal, one dark and one light)
but you can also do it using one picture - by creating the light and dark ones using the normal picture
this might be a bit overdone and not look real but shows you what i mean: