The operating system is free to use and everyone has the freedom to contribute to its development. This co-operative development model means that everyone can benefit. Because of this, we like to call it Free Software, or Socially Responsible Software. Closely related is the concept of Open Source Software. Together, Free and Open Source Software is collectively abbreviated as FOSS. This contrasts with the proprietary (or closed source) development model used by some software companies today.
Linux has many other benefits, including speed, security and stability. It is renowned for its ability to run well on more modest hardware. Linux comes from the venerable UNIX family of operating systems, and so has been built from the ground-up with Internet-style networking and security in mind. Hence, viruses, worms, spyware and adware are basically a non-issue on Linux.
Linux (also known as GNU/Linux) is a computer operating system, like Microsoft Windows or Apple Mac OS. Unlike those two, however, Linux is built with a collaborative development model. The operating system and most of its software are created by volunteers and employees of companies, governments and organisations from all over the world.
Many of the principles behind FOSS are derived from the axiom of standing on the shoulders of giants, most famously used by Isaac Newton, which has guided scientific and industrial development for hundreds of years. Transparency of the code and development process means that it can be participated in and audited at all levels. Software is just another form of information, and people have the right to have full control over that information. In the same way that you are free to share cooking recipes with your neighbour, you should also have the freedom to share and change software.