As you may know, Kali Linux (formally known as BackTrack Linux) is among the most popular penetration testing operating system available.
The latest version available is Kali Linux 2.0, which is powered by Kernel 4.0, uses Debian Jessie as source-code and GNOME 3 as the default desktop environment, while KDE, XFCE, MATE, Enlightenment E17 and LXDE are also supported.
Also worth mentioning, starting with Kali Linux 2.0, the system will be a rolling release model, meaning that the users will be able to easily upgrade to a new system.
If you have a previous Kali Linux system, this is how you can upgrade to Kali 2.0:
$ sudo su
# cat << EOF > /etc/apt/sources.list
deb http://http.kali.org/kali sana main non-free contrib
deb http://security.kali.org/kali-security/ sana/updates main contrib non-free
EOF
# apt-get update
# apt-get dist-upgrade
# reboot