Linux is an operating system1 just like Windows or OS X. You can use it to browse the web, play games, and much more.
Linux comes in many different versions, called distributions. Distributions come in all shapes and sizes and most are free of charge.
Linux is highly customizable due to its modular design. There are many tools available to make your system your own.
Steam runs natively on Linux with over 1500 titles.
The LAMP stack is the platform of choice for programming robust, modern web applications.
1Technically Linux is the kernel - the core of the operating system which provides an interface between user applications and hardware. Linux distributions consist of the Linux kernel with other software packaged: most notably the GNU system.