Linux is the core part or 'kernel' of many free and open source computer operating systems. While there are many operating systems which run on different types of computers, the Linux-based family of operating systems can offer much to the casual user or professional alike.
Linux is free
By 'free' I mean that the source or programming code is published under licence terms which are favourable to the user. The user will always have access to the source code hence they are free to use, change, and redistribute the software so long as they comply with some simple rules, none of which are likely to limit the freedom of the average user. Most users will happily use the software and never worry about delving into the source code but it is nice to know that you have freedom to do so if you have the need or the skill.
Most versions of Linux that you can install on your personal computer are also free in a monetary sense or at least they typically have functional, free versions that you can use with few restrictions. So what does all this freedom get you?
Linux-based operating systems tend to be secure from the outset as normal users are permitted to perform certain non system-critical actions only. To change, delete or otherwise meddle with important system files you need to type in a password. Even if your computer is compromised this step gives is an extra degree of protection. While no piece of software is guaranteed free from bugs or viruses, the Linux kernel and most pieces of Linux software are developed in the open hence bugs and vulnerabilities for important software are identified swiftly and fixed rapidly by the software developer or maintainer. By contrast, a 'closed source' approach to software development generally means that the developers do not release the source code so there may be fewer opportunities for security flaws to be identified and fewer people to solve any issues identified.
Linux-based operating systems are used routinely to run a large proportion of servers on the internet. There are large companies or communities which support these operating systems and which make sure they are reliable and stable. There are many related desktop versions of Linux which have many of the same characteristics which can be used for everyday computing tasks.
For many common computing tasks there are simple tools available to the Linux user. With some rudimentary programming skills an enthusiastic user can string together instructions to take advatage of these simple, modular tools to undertake potentially complex computing tasks which on other operating systems may be more difficult or necessitate buying software.
Many supercomputers run Linux as it can be readily modified and re-purposed to optimise performance on a wide range of types of computer. Linux supercomputers help forecast the weather, serve your e-mail and crack scientific puzzles.
Linux and fun
While Linux can be as simple to use as many of its more popular competitors it also offers a wealth of opportunities for learning about how computers work. It is an excellent learning environment.
For those with zero aspiration at becoming proficient in Linux system administration, Linux has fun, free games to pass the time. If you want to learn more check out Ubuntu which is one of the most popular and user-friendly versions of Linux available.