Linux is not the name of a specific and full working OS, but implements only the essential inner core of an OS, which is referred to as the kernel. Most types of Linux OS do not cost anything, but provide thousands of programs and software to use completely free of charge. Most of these programs are also open source, which means you can view the exact blueprint of how the program has been created and it can be changed by anyone. One very important area in which Linux is very popular and is used heavily is managing many network services. These are programs that run in the background of your Linux server, continuously waiting for external events to respond with some kind of action or information. Examples are popular internet services such as a web server, which presents websites to the user; main servers for email communications; and also database servers to store and deliver any kind of data. As mentioned before, Linux is only the name of the kernel part, not a full working OS. To make it complete, you need to bundle it together with all kinds of programs, which is then referred to as a distribution.
Nowadays, one can choose between an insane amount of different Linux distributions, all designed for a special purpose and all with their own pros and cons. The main difference between them is the software selected and bundled together with the Linux kernel. The most important Linux distribution families are Red Hat- and Debian-based Linux distributions. CentOS is one of the most important free Red Hat-based Linux server distributions out there at the moment. It's a very stable, secure, and reliable OS, which is why it's often used in running very critical networking services in enterprise environments. Also, it's good to know that the development of this OS is very strong and updates are well selected and suitably tested. The first chapter of this book is about installing Linux; here we will be starting easy by introducing you to the concepts of virtualization. Then, we will create a new CentOS 7 virtual machine (VM) using a free virtualization software called VirtualBox.