JTR is short for "Jack The Ripper", its a hash cracking tool used by beginners and veterans alike to crack password hashes on unixlike systems.

I have also used this tool to crack the NT SAM hash manager on windows NT (NTFS filesystem) based systems. There is also another tool that is also great in cracking NT passwords, but I digress.

In my spare-time, I enjoy cracking my own systems to see how far I can get without using a password. Its pretty amazing what you can learn by doing these kinds of things.