By default, you're not allowed to become root on the N900.

To do that, you must first install a package called "rootsh".

After that, you may type "sudo gainroot" to become root. At that point, if you like, you can set your own root password. This would allow you to type "su" and enter your root password to become root, like in Suse.

"sudo" is used to allow a user to execute a command as THEMSELVES, but with root privileges. It is also available on Suse, and all other unixes.