what is the SU password for the terminal window?

That is the root password. When you do the install you create one account and put a password used like root password

Type sudo -i enter your password when asked and you will get to a root session. I discovered that you can then change the root password to whatever you want with the passwd command and su will work as normal. I haven't done this on my latest install and haven't had any problems using sudo when needed though.

The Ubuntu system is "rootless". There is no active root account in the livecd or the default install. Instead of that, the first user you create while installing have permissions to act as root through "sudo"

Advantages of sudo is that you can grant access to "User A" to some root actions, "User B" no root privileges, and "User C" all access, and all that without giving them the "root password" as you must do with su. They all use their own password.

At the same time you don't need to start a subshell with root privileges to run one command and then exit to the main shell. Instead of that you do "sudo command", enter your password and the command is executed with root privileges (if you are allowed to) and when done you are on your limited user shell. That way you don't forget to exit from the privileged one and can't do something bad by mistake (or at least have less opportunities).

The following explains this in detail:-

http://wiki.ubuntu.com/RootSudo