BackTrack is a distro build around (a lot of) penetration testing software.
You don't "learn BackTrack", you need to get acquainted with (some of) the software that is available. BackTrack offers a wide range of software (which are divided into something like 10 categories), you probably don't even need all of it, just the once that are of interest to you (penetration?, forensics? to name just 2).
I would advise to visit the websites of the specific piece of software you want/need to use (nmap for example: nmap.org
Hope this helps.