Linux is a modern, mature, free computer operating system with a graphical user interface very similar to either Windows or Mac; your choice. In fact, Linux is about choice--and freedom. Best of all, you don't even have to install Linux to try it out. You can just pop in a "Live CD" reboot your computer and be running Linux in minutes. (with no danger to your existing operating system). You can even get an inexpensive CD shipped to you. Actually though, you're probably already using Linux and don't even realize it.
Linux originated in 1991, but is now reaching a critical mass of adoption with millions of users worldwide. Most corporate uses are for servers, but it is now increasingly being used on the desktop by businesses and ordinary computer users too. Even Apple users are impressed with how usable modern Linux is. Among Linux users are major corporations including IBM, HP, Sun Microsystems, Google, Dell and Novell. Government agencies like DOD, NASA, NSA, and the US Army use Linux. The world's fastest computer runs Linux. Many cities, medical facilities, educational institutions, airlines, the NYSE and foreign governments use Linux. You should too.
Instead of having your use of the computer you bought dictated by a giant corporation (and convicted monopolist), YOU can be in charge! Linux gives you freedom; free from cost, free from spyware, adware, worms, Trojans and virus infection. No virus scans, no malware removal, no defraging to waste your time. Oh, and you don't need to buy any expensive third-party utilities to protect yourself either. Linux is secure out of the box. And, many manufacturers try to make up for the cost of a Windows license by bundling demo and trial versions of software such as AOL, Norton, McAfee, Microsoft Office, etc. (known as "crapware"), for which they receive money from software companies looking to increase their sales. These bog down your system and are hard to remove. Not on Linux. You start with a clean system.
Another advantage, Linux runs well on slower computers, so you can save even more money by upgrading your hardware less often, or buying cheaper systems in the first place. Not only do you not have to pay the "Windows Tax" when you switch to Linux, you have access to thousands of pieces of application software as well; Office Suites, a Photoshop replacement, audio and video editors, multimedia players, and on and on. Free.
One more thing; you can update your whole system, both the operating system and all the applications with one command! Likewise, in some distros you can upgrade to the latest version of the operating system (and all it's programs) easily. For free.
Linux also gives you freedom from onerous EULA's and entering long cryptic codes to install software you paid for. Quit worrying about product activation, how many computers you can legally install the software on, planned obsolescence, forced upgrades, vendor lock-in, DRM, and all the other problems from the closed software model.
But Linux is not a copy of Windows. It has it's own unique advantages and it must be said, some disadvantages. Also, Linux and Free Software are as much a philosophy as a technology, but many people misunderstand the term "free software":
"Free software is a matter of liberty, not price. To understand the concept, you should think of free as in free speech, not as in free beer." gnu.org
(A similar term, Open Source, is slightly different.
For one point of view on the differences, read this essay)
In most cases though Linux is "free" in both senses of the word. Freedom for free--you can't get much better than that.
Freedom feels good. Give it a try.