I believe that 100% of all people who have used a computer can say that they have used a PC before. This does not mean that a PC is the only kind of computer out there. It also does not imply that PC is the only kind of computer that those people have ever used. While Microsoft's PC (Personal Computer) is by far the most widely known and used computer in the world, there are other company's out there that are catching up with Microsoft.
An OS (Operating System) is what a computer operates on. For example, Microsoft has been developing new OS's has time goes by and technology develops. The newest OS from Microsoft is “Windows Vista”. Other OS's from Microsoft before vista include “XP”,”2000' (or Millennium), “98” and “95”. Chances are right now you are reading this on a system running XP or Vista. But what other kinds of OS's are there? Right now, the highest competitors, respectively, are Macintosh (Mac) and Linux.
Mac is an entirely new concept to the world of computers. But that doesn't mean their hard to learn how to use .The Macintosh system is developed by the company “Apple”. Apple is to Mac as Microsoft is to Windows. Apple is the developer of the most popular hand-held gadget today. It has passed up the popularity of a cell phone in only six years of existence. This device is the iPod. This famous gadget released in 2001 originally played music and had an amazing amount of storage (at the time). The original iPods had a capacity of around 10-15 GB (Gigabytes). At the time, this was a huge amount of capacity for such a small device. One GB is the size required to hold about 250 songs. 3,500 songs in your pocket was really quit a revolution in 2001 (It still is today).
The iPod has brought Apple a lot of exposure in the past six years. Since they decided to make the iPod compatible with Mac's and PC's, The Mac is becoming increasingly popular at a steady rate. A Mac does have a few large differences than a PC. But people who have used both systems and own a Mac will swear their life on that Mac. I have never heard a story where somebody bought or used a Mac and didn't love it. The first recognizable difference is that that familiar “Task bar” with the big start button on it is gone. But a familiar bar is replacing it at the top of Mac's desktop. The other thing about Mac is that almost all of their compatible software looks exactly the same. So if you've used iTunes 7 or later, then you know what a Mac looks like.
The last OS that I'm going to mention with growing popularity is Linux. Linux has a strange sort of OS. It really doesn't have one updated OS, but many flavors of OS's. Right now they have about five or six different OS's but they are all very close to one another. The most common one, and in my opinion, the best one, is Ubuntu. This is a screenshot of my Ubuntu software. This is what the desktop looks like. Ubuntu has automatically put a link of my Windows hard drive on the desktop. (I enlarged the link for this image so that you could see it better. The actual links are not that big!)
The best part about Ubuntu is that not only is it an amazing OS, but it's completely free! You don't have to pay $200-$300+ just for the Os. It's 100% completely free. You will also never pay for pricy software if you use Ubuntu. Microsoft office can cost as much as $300 just for five or six programs like word, PowerPoint, excel exc. Ubuntu has the equivalent of these programs absolutely free! And in my opinion, these programs are just as good as Microsoft Office, if not better. Ubuntu has a growing community of users who are always friendly and are glad to offer free support. Ubuntu also has thousands of free programs that aren't just little cheap, user created programs. All of these programs are high quality, functional programs. SO what does that mean? You can download a copy of Ubuntu and install it absolutely free! Once you do this, you can begin browsing the internet for all of you free software. You don't even have to go to an internet browser to find these files. Just go to your desktop, then click Applications and in the pop-down menu, click Add/Remove.
This is another excellent and free program. It has many categories on the left-hand side of the window. Just click your category and type in the search bar what you're looking for. It's that simple. And, once again, it's all free! 
Decent article.