Kali Linux is an open source project that is maintained and funded by Offensive Security, a provider of world-class information security training and penetration testing services. In addition to Kali Linux, Offensive Security also maintains the Exploit Database and the free online course, Metasploit Unleashed.
Side note, I've use Kali Linux and Backtrack. I really suggest this to everyone in networking and such. I like it and enjoy the OS.
I still prefer and use Backtrack 3, but I have downloaded Kali Linux to test it out a bit. Again, they've shoved it packed full of goodies for network administrators and those who like to fuck shit up.