Kali Linux (successor of BackTrack Linux) is a free and open-source Debian-based Linux distribution funded and maintained by Offensive Security from Switzerland. Users can boot and use this OS via hard disk, live CD or live USB anytime and anywhere. Almost all open-source tools used for digital forensics, penetration testing, ethical hacking and network security assessments are included. There is no need to prepare and maintain this Linux distribution (including gathering all those software and their dependencies) by yourself, or to install any software and save any files on Kali.

MOFO Linux was originally designed as one portable and bootable Linux distro (developed on the basis of Ubuntu, can be installed on HDD, USB flash drive, SD card or burned into Live DVD/USB using the included Startup Disk Creator, UNetbootin, or "DD copy method") that can be used on public computers for protecting user privacy. Gradually, it developed into one Linux operating system that emphasizes multimedia, VPN, proxy, file encryption, and disk wiping features.

Linux is cool not just because it's free and open-source, more often than not, that's because it's so powerful. For example, it is used to build large servers with high performance and high stability; to make the old computers bloom; as desktop or office operating system for individual users or enterprise users, and more. I think any users who are interested in computers, somewhat get tired of Windows, and want to try some new OS, they may want to begin to reach the mysterious Linux operating system. But if you want to make learning Linux easy, you really need a good book for the level of beginners.

CentOS (short for Community ENTerprise Operating System) is a free, open-source, enterprise-class and community-supported computing platform that derived from the same source code with Red Hat Enterprise Linux OS. At present, there have been increasing Internet services or applications around the world that require high stability (including a variety of purposes of servers and business applications) are running on it, rather than RHEL.

As an open-source and UNIX-like OS, Linux is so powerful and highly secure that it has been controlling so many IT technology fields of great concern. Therefore, mastering the essential basic knowledge about Linux can make IT professionals scale new heights in work.