An Operating System (abbreviated as OS), is a software that is responsible to manage the system resource. An operating system contains many applications in it. Operating system is directly handle the request which are received from Hardware devices (i.e. Keyboard, Mouse, Motherboard etc.).
There are many operating systems provide GUI (Graphical User Interface) through which you can manage it and some others provide CLI (Command Line Interface). It is easy to manage the Operating system which provides GUI. For CLI operating systems, you should have strong knowledge of commands.
Now a days, most cmmonly used operating systems are Windows, Linux, Unix, Solaries, CentOS etc. Some of them are commercial version and you will need to purchase the license to use it.
Linux is the open source and hence it is freely available. However if you want enterprise edition of Linux then you will need to purchase the lincese for Commercial version.