Kali Linux, previously know as Backtrack is a security distribution created by some very clever people at Offensive Security. It is supported on many different platforms from classic x86 computers, tablets, smartphones and even arm-based systems like Raspberry PI.
Kali contains a tremendous number of security tools for every stage of penetration testing. From service fingerprinting, enumeration, password cracking, wireless attack tools, sniffing, spoofing, reporting and the list goes on an on.
Knowing these tools and techniques not only re-enforces your knowledge about security, but also helps you get the necessary hands on practices so you will know how to protect your environment from real threats.
There are excellent number of resources located that offensive-security site. They have articles, forums, free videos and top paid courses. I once gone through online course with Mati Aharoni aka. muts who demonstrated how real penetration testing is done. At that time it was a huge inspiration for me, and it was also the same time I discovered an amazing band called Infected Mushrooms and their mind-blowing song Suliman.