Password cracking is a fundamental hacking and digital forensics skill. Nearly every system is authenticated by the traditional username and password and even if two factor authentication is used, one is usually a password.
Cracking passwords can take many forms. See my article of password cracking strategies before you embark upon password cracking. It will likely save you significant amounts of time.
For Wireless (Wi-Fi) Password Cracking, check out my Wi-Fi Hacking series.