Hi, welcome aboard!
First, Ubuntu isn't another form of Windows. It is a completely different way of life. It's as different from Windows as iMac is.
That said, once you get the hang of it, you'll find that it's fairly straight forward.
In terms of software, unlike Windows where you download from the net, Ubuntu uses 'repositories' which can be accessed through the Ubuntu Software Centre.
Windows software, by itself, wont work with any Linux distribution. It's just not designed for it. There are ways around this, but nothing is guaranteed to work. For the most part, there's usually a suitable Linux alternative in the software centre. Increasingly, more Windows developers are designing for both Linux and iMac's now, so who knows, maybe your fav software already has a Linux version.
As far as customisation goes, being open source, you pretty can pretty much let your creativity fly!
My advice would be to download Ubuntu onto a usb stick, and play with it in live mode, before installing it on your PC.
Like I said, welcome aboard!
Windows free since '09 - never looked back!