Heh,
I don't have a Zaurus *yet*, but I'll do my best to help you.
1. A ROM is kinda like the Operating System on the PC (Windows, Linux, Mac OS).It's basically a Kernel (Tell ya more about the Kernel Later), a GUI (Graphical User Interface, or what you see on your Zaurus when you use it, except when your screen is full of text), and a bunch of Applications.
2.Qtopia is the GUI used in your Zaurus ROM (We call it the SHARP ROM).
3.Cacko is a alternate ROM.So are the PdaXrom, Openzaurus, Watapon and a million of other ROMs.Check their respective webistes for more info.
4.Develop software, of course! Well, If you've heard of C++, Python, Java and all that, the Zaurus can make programs, if you know how, although the Zaurus is not powerfull enough, compared to a PC.There are millions of books out for learning programming!! Search Amazon, or Google for more!
5.Now, all Kernels are basically the same, just their versions are diffrent.There are two out right now, the 2.4 family, and the 2.6 family.If you want more info on what a Kernel is, look here:
http://en.wikipedia.org/wiki/Kernel_%28computer_science%29The Kernel is basicly the piece of Software "talking" to the Hardware, and telling it what to do.You get the idea.
The ROMs out are:
http://openzaurus.orghttp://www.pdaxrom.org/http://my-zaurus.narod.ru/cacko.htmlhttp://www.handhelds.org/moin/moin.cgi/FamiliarDistributionhttp://zaurus.duelmonsters.ca/http://www.pocketworkstation.org/Now, none support your device, the C3000, which means that if you try to put them in your device, you're in trouble.Now, the BEST ROM is clearly a personal opinion, but in my opinion the most mature projects are the OpenZaurus and the PdaXrom, and mabye the Familliar ROMs.
That's all folks!!