Linux is an operating system, just like Windows or MacOS X. It allows you to work as you would on Windows. But it works differently.
Some still see Linux as a horrible system full of complicated commands. This is no longer true. Linux now has a nice, comfortable and easy-to-use Windows graphics system.
We used to say that Linux is an operating system. In reality, Linux is only the heart (we say the “kernel”) of the GNU / Linux operating system.
(Microsoft Windows NT, 2000 and XP operating systems also have a kernel, but different: the NT kernel.)
The kernel takes care of the basic tasks: memory management, access to peripherals (hard disk, CD-Rom drive, keyboard, mouse, graphics card, etc.), network management, microprocessor time sharing between programs (multi-task), etc.
Unlike Windows whose graphical interface is imposed on you, there are different graphical interfaces under Linux, the main ones being Gnome, KDE and XFCE. It’s even possible to run Linux without a GUI, or even launch the GUI only when you want.