There are thousands of differences between mac and windows operating systems. To try and explain everything would take me all day and a books worth of typing. I can however explain the primary differences. First Windows is based upon a DOS programming language (think of it as the foundation for the operating system); Mac is based upon unix programming language, (linux is also based upon unix as well.)
Macintosh is often more secure than windows due to less attempts at hacking because the majority of computer users use Windows.
Macs tend to focus on graphics and multimedia while Windows focuses on office function.
Benefits of Mac:
No viruses, or any other malware for that fact. (This is due to Mac OS X's UNIX underpinnings, not "lack of users" or "low market share.")
Seamless integration. Your hardware will work perfectly with your software, because both are designed by Apple.
You will enjoy many advanced features that are not available in Windows, even Vista. Features like Exposé, Spotlight Search, a built-in dictionary that can be invoked in any application with a simple keystroke, easy to build scripting using Automator, zero-configuration networking, system wide spell checking and many more.
Macs can dual-boot with Windows or run Windows within Mac OS X itself. With BootCamp, you can dual-boot with Windows and run any PC application, even high-end games.
Excellent support and customer service. Apple is well known for it's support. I had a wonderful experience with Apple when the logic board in my Mac mini went bad. (This was due to a power issue in my home, not the Mac.)
Great software. Most of the software made for the Mac is better than similar software for Windows. Crap doesn't survive on the Mac market. iLife is absolutely amazing. Mac OS X is compatible with many file formats, right out of the box. As I mentioned before, you can run Windows applications. With a Mac, you can have it both ways.
Beautiful hardware. Quality hardware. Apple puts the sex in tech. Apple hardware is always beautiful, as well as functional. Apple has extremely talented engineers. One look at an iMac will tell you that much. The hardware is of excellent quality and built to last. Macs are well known to have a longer life span than PCs.
Upgrades? Who needs them. A seven year old iMac can still run the latest release of Mac OS X. Sure, you can add more RAM or get a bigger hard drive, but your Mac will run beautifully for years to come. When a new version of Mac OS X is released, you won't need to scramble to purchase upgrades in order to run it. (Leave that to the Vista adopters.)
Crashing. It doesn't happen. I'm not saying Mac OS X is absolutely perfect, but it's damn close. Most problems are easily fixed within minutes. Macs don't require the constant maintenance needed by PCs.
Value. When all of these things add up, you'll actually be saving money, as well as time, in the long run. You'll find yourself actually doing more on a Mac. Things you never thought of doing on a PC.
