At the top of this page it says that if you are new to Linux and want to become proficient with BackTrack you should first acquaint yourself with Slackware Linux because BackTrack is based on Slackware.

However, as far as I know BackTrack 4 will be based upon Debian Linux. By the time I learn to use Linux in general, BackTrack 4 will probably be out (I'm guessing mid-summer 2009), so my question is - should I be learning to use Debian Linux now instead of Slackware Linux if I want a smoother transition to BackTrack 4 in the future? How much difference is there between Debian and Slackware?

Also, a separate question is what does it mean that BackTrack is "based" off a certain other Linux distribution? I understand that different distribution are all based on GNU/Linux, but bring their own touches to the overall OS. Is BackTrack just a slimmed down and IT-Security optimised version of another Linux OS (Slackware or Debian, whichever it may be)?