Working in the internet field, sometimes I need a good description of how something works to pass on to a client. Yesterday, one of our network techs pointed me to Webopedia: Online Dictionary for Computer and Internet Terms. Great information, easy to find things, and I've already used it to explain to a client what XML means.
I have also used WhatIs in the past, but the search in PCWebopedia was faster, and the information seems to be more concise.