Architecture of Linux Kernel.

About Linux

The Wikipedia article about Linux:

Linux (commonly pronounced /ˈlɪnəks/ LIN-əks in American English,^[4][5] also pronounced /ˈlɪnʊks/ LIN-ooks^[6] in Europe) refers to the family of Unix-like computer operating systems using the Linux kernel. Linux can be installed on a wide variety of computer hardware, ranging from mobile phones, tablet computers and video game consoles, to mainframes and supercomputers.^{[7][8][9][10]} Linux is a leading server operating system, and runs the 10 fastest supercomputers in the world.^[11]

• Its a Unix family operating system.
• Linux has largest kernel ever.

Over 1,000 developers, from at least 100 different companies, contribute to every kernel release. In the past two years alone, over 3,200 developers from 200 companies have contributed to the kernel--which is just one small piece of a Linux distribution.

An abbreviated list of some of the popular electronic devices Linux is used on today includes:

Dell Inspiron Mini 9 and 12	Garmin Nuvi 860, 880, and 5000	Google Android Dev Phone 1
HP Mini 1000	Lenovo IdeaPad S9	Motorola MotoRokr EM35 Phone
One Laptop Per Child XO2	Sony Bravia Television	Sony Reader
TiVo Digital Video Recorder	Volvo In-Car Navigation System	Yamaha Motif Keyboard

The term "Open Source".

An Open Source software or operating system means, whose source code is openly available (mostly not in the case of Windows!) to all, and which can be modified by anynody for any purpose.
That means, such types of softwares/OSs do not cost you anything, almost. Most of the versions of Linux OS can be ordered for free. They are freely distributable.
Free distribution leads to freedom of its use. No piracy!

And Linux is so handy operating system with open source, that it has attracted us to see to it.