Linux is just another Operating system much like Windows or Mac. The biggest difference between the three is the fact that Linux is, in most cases, free, and to the best of my knowledge is always open source. The only Linux system I've used is Ubuntu, and I love it. It's much faster than Windows 7, which I have found to be about as fast as Mac.
The trickiest part of Linux I've come across is getting your typical programs to work. It can be done, but for me there's always been an alternative which again, is free. For example, replacing the Microsoft Office programs (Word, Excel, Outlook, etc.) you have OpenOffice.org. Completely free, and pretty similar. Not as powerful as Microsoft, but for the typical computing, it's plenty.
For a little more information on Ubuntu:
Features | Ubuntu
Ubuntu is nice because you can actually install it
inside of Windows, choose to boot into Ubuntu any time you want simply by restarting your computer and selecting Ubuntu when it asks you which OS, and then if you need to free up space, you can just uninstall instead of deleting the partition, which in some cases has been known to make your hard drive act goofy.
