1) Rooting is basically installing an altered version of the nook firmware so you have root, or full, access to the nook. It allows you to install other apps, use coverflow on your sideloaded eBooks and much more. More information can be found at
http://nookdevs.com/Main_Page
3) Sideloading means putting books onto an eReader in the "my documents" folder on your nook. If you use calibre, the files you send to your nook from there are being sideloaded from. Barnes and Noble NOOKbooks are not sideloaded.
And unfortunately I don't have an answer for the other two questions, but I'm sure someone else will have one for you soon
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