SU and ROOT - The difference

There have been questions , what is the difference between su and root . . well, the difference is minimal but still very important.

As you login as root right from booting the system, all the privileges are set to root, what makes your system more vulnerable to your own mistakes and those wishing to enter your system.
As you login as normal user, the privileges are limited, thus its safer. To be able to do administrative tasks you can temporally login as root using su in a console/terminal, that way the root privileges are limited to the actions you do in the console ( and the applications you start from the console ). So the rest of the system is still in normal user mode, and this makes it safer.
We do use the same password for su and root, but once you are logged in as normal user, typing root in console/terminal will only give you bash: root: command not found.