A dep is a mode where you can use the EOS to compute the aperture needed to have the correct deep of field.
Imagine you have 3 matches in front of you :
One a 1 meter (or yard or whatever unit you prefer) another at 2 meter and the last of the 3 at 3 meter.
If you want all matches to be on focus you have either to set you camera to Aperture mode "guesstimate" the correct aperture to have a deep of field making all 3 matches focused ,use the DOF tester on you camera (and notice your guess was wrong)
In short a mess for lazy people like me
In ADep mode you focus on match1 ,next on match 3 , the camera will compute the optimum aperture ... obvioulsy you shot fosuing on the 2nd match
check this :
http://www.vad1.com/photo/autodof.htmlit's perhaps clearer than my post
(I'm heading to bed)
@Toronto : made some photo today I noticed that my 350D allways overexpose in result program when it's allway correct in Tv/A mode ...