There's " linux the operating system," which normal users don't see, and the UI which is how most people relate to linux. Android and WebOS use "linux the operating system" but put their own UI on top. maemo has some custom UI components but shares a great deal with desktop machines. Sharing UI components makes it much easier, in some cases trivial, to port software from a desktop environment to maemo.

Some of the capabilities someone might like to add to maemo require administrator access, which is called "root" on linux. On linux you don't run as administrator usually to a) make it more difficult for the user to screw up the machine b) make it more difficult for other people to screw up the machine.

Just a couple of days ago I paired my wiimote with my n800 so that I can control the media player more safely while driving. https://garage.maemo.org/projects/wiicontrol/ I don't recall if that requires root access, but on a linux netbook I used admin access to use the wiimote as a mouse. This was different software though. When I get my n900 I hope to be able to use that with tv out though admittedly I have not looked up how to use tv out yet.

This is not something you can do on most cell phones without voiding the warranty which is why sanctioned root access is a big deal.