As we know Kali Linux is the successor of BackTrack 5, so there are much similarities to BackTrack in many ways, but it lays a new foundation and makes substantial improvements that will allow it to be even more useful to penetration testers in the coming years.
Many security practitioners have been using BackTrack to perform their security assessments. BackTrack is an open-source Linux-based penetration testing toolset. BackTrack made performing a security assessment easier because all of the common tools that you needed were all packaged into one nice distribution and ready to go at a moment's notice. BackTrack contains so many security tools that it is too difficult to list them all. If you want to see a complete list of all the tools just run "dpkg --list". BackTrack made it easy to create a new VM from the downloaded ISO, perform the assessment, then either archive that VM for future reference or delete it when done to remove the evidence. The latest version of BackTrack is version 5 release 3 and it came out on August 13, 2012. If you want to explore this version, you can download it from this site. But before, you do, check out Kali Linux first.
Kali Linux is a new open source distribution that facilitates penetration testing. Whereas BackTrack was built on Ubuntu, Kali Linux is built from scratch and constructed on Debian and is FHS-compliant. Kali Linux also has improved software repositories that are synchronized with the Debian repositories so it makes it easier to keep it updated, apply patches and add new tools. It is also easy to customize your own Kali Linux so that it contains only the packages and features that are required. You can also customize your desktop environment to use Gnome(default), KDE, LXDE, XFCE or whatever you prefer.
The development of Kali Linux was funded by Offensive Security. Offensive Security is a security training and penetration testing consulting firm that has been a creator, supporter and maintainer of BackTrack since the beginning. For years they have offered their popular Penetration Testing with BackTrack (PWB) class, but with the introduction of Kali Linux, that class name will likely change.
To get your hands on the latest Kali Linux distribution, you can download it from: http://www.kali.org/downloads or http://cdimage.kali.org.