Answer:
Linux is usually free and is freely distributable. The source code can be changed, copied and altered, giving you freedom.
Unix/Linux is trusted for mission critical applications (file servers, workstations, supercomputers) - it is also widely adaptable. Linux will work on devices from cell phones to supercomputers, such as the IBM Roadrunner (which runs Fedora Linux).Remember that Unix is like Linux, but they aren't the exact same thing.
Linux is more secure. Many viruses are written for Windows (about 99%). Linux has security built-in to its kernel. Microsoft Windows does, but it's not good enough. Many users of Linux attest to the lack of viruses, hacking attempts, and other computing annoyances.
Linux is actually a mature technology. Its roots are in Unix, a technology over 30 years old. Unix was and still is superior to any other operating system. Unix comes in many different flavors - BSD (Berkely Standard Distribution), Solaris, AIX and MacOS X are most popular modern Unix flavors. Linux might as well be Unix because it is strikingly similar in its operation and its design of the kernel, but Unix is a proprietary technology.
Linux runs well on older hardware. I have a Pentium III with Zenwalk Linux running at 600 Mhz with 320 MB RAM running alongside my much newer Athlon X2 running Ubuntu and Fedora (dual-boot) with 2 GB RAM and my ThinkPad with a Core 2, which runs openSuSE. Most people, especially those who use Windows, would junk the old Pentium III but it serves me well as a small, simple home file server which runs like a top. Windows XP would do okay on a machine that old, provided you did relatively simple tasks such as word-processing and maybe web surfing but with Vista starting to dominate, a machine that old would be worthless unless you use a Linux distro on it.
Linux is very fun to use! Very quick, agile, and eye-appealing desktops (GNOME, KDE and others), and then you have the simple yet powerful command-line. If you try just a couple of commands and practice - I would probably find a unused PC to do this so that you don't accidentally cause damage - you would excel at the command line. The command line does what the desktop can't. For example, if I'm feeling lazy but I have a program (package) I just have to have, I can just type sudo apt-get install {program name} and I have it in a couple seconds or just a quick minute or two - without shuffling through window after window to get to it (I find that it is much quicker and actually easier to use the command line for system tasks.)
Linux is starting to become more popular. Why? Well, a lot of netbooks (very tiny laptops) which are very popular are flooding the market. Most of those have Linux. Also, now that XP is starting to be yanked off the shelves and Vista is taking XP's place, people who have heard the horror stories about Vista are looking for alternatives. Linux is slowly but surely replacing Windows in some businesses and large corporations and almost anyone who works with networks knows that basically your only choice for a NOS (network operating system) that won't ruinate your company is Linux. Also, more people are just simply hearing about it, getting the guts to try it, and never want to go back to what they were using before. I suggest that you give it a try - don't worry about it being hard to use (it's not) and worrying if your files won't work with Linux programs - it will, and if it doesn't out of the box, a visit to a support site or forum can fix that.