There are a lot of free (or pirate-able) documents for Kali Linux if you're interested in learning more about it. I'd recommend "Kali Linux - Assuring Security by Penetration Testing" by Lee Allen or "Mastering Kali Linux for Advanced Penetration Testing" by Robert Beggs.

Kali is almost identical to BackTrack, it is "the rebirth of BackTrack". From my playing around it has all the same applications installed but is on a new kernel and with update apps. They really do appear to be continuing the legacy of BackTrack