It's an open-source Unix derived kernel, that many operating systems use. You may have heard of some such as Ubuntu, Fedora, RedHat, CentOS, Mandrake/Mandriva, Lindows/Linspire etc...
It's the most prominent server kernel in use today and has been for many many years, due to its relative security and stability compared with the Windows NT kernel amongst others but similar flexibilities and support for web fomats and tools, but it's particuarly good for networking, and you can get software for it that when paired with certain 802.11 chipsets, can easily scan and crack Wifi networks, regardless of WPA or WEP.
I use it all day as part of my job, but would'nt really recommend it for n00bs as its quite a bit different from Windows (its easiest to bypass the GUI and control everything from the command line). If you have the time and think it's worth the effort though, you could do a search online and find out how to crack wireless networks using "Live" version of Linux OS's - I can't really post the info on Wiki because let's just say, you can take control of people's networks and even computers etc with the right software and knowhow. Redirecting someones browser to naughty sites everytime they visit facebook.com for instance, is a bit of fun but only takes seconds to achieve.
But the most useful resource for anyone without mallicious intentions, is of course, using their internet connection. Parking up outside a random persons house and using their internet connection when you need to - we call it "wardriving".