John the Ripper is a fast password cracker, currently available for many flavors of Unix, Windows, DOS, BeOS, and OpenVMS. Its primary purpose is to detect weak Unix passwords. It supports several crypt(3) password hash types commonly found on Unix systems, as well as Windows LM hashes. On top of this, many other hash types are added with contributed patches, and some are added in John the Ripper Pro. 

The goal is to unify all of the good information found in various bits and pieces into 1 large document. This document is for people who want to learn to the how and why of password cracking. There is a lot of information being presented and you should READ IT ALL BEFORE you attempted doing anything documented here. I do my best to provide step by step instructions along with the reasons for doing it this way. (SOURCE : Jason's 'Cracking Password Techniques')