What version do i need for Windows 8?
If you have Windows 8, you can either install it alongside in a separate partition by downloading the desktop version, or use the Windows Installer. Either way, if your computer came with Windows 8 (rather than you installing it yourself) you should go for Ubuntu 12.10 or 12.04, nothing earlier.
What happens if it doesn't work?
You can ask questions here, or take to the forums to find solutions to your problem. If you're not satisfied with something, do the same thing. Chances are, someone's of the same opinion, and has made an alternative!
Unless you buy Ubuntu Advantage or Ubuntu Advantage Advanced, Canonical (the creators of Ubuntu) won't be providing you with any direct support, although some of their developers do volunteer on here as well.
What about Customer Satisfaction?
Remember that this isn't a paid product, it is something you get for free. It doesn't come with a warranty or guarantee in the traditional sense, and you can get rid of it whenever you want.
Is this Linux?
Linux is a free, open-source kernel, the very base of your operating system, and is what Ubuntu uses. More commonly, Linux is used to describe (sometimes incorrectly) any open-source operating system. Either way, yes, Ubuntu does boot from a Linux kernel, and is an open-source operating system.
So i don't need to purchase anything?
No, you can download, modify and share Ubuntu as you please. The DVDs are there mainly for people that either want a more professional touch, or perhaps have severe bandwidth limitations.