I'm not sure that you have got what you are actually doing; apologies if the following is obvious to you and I'm reading too much or not enough into your question.
sudo passwd sets the password for the user
root. Once you have done this, you can log into your machine with the username
root and the password you have set (in a terminal anyway, it is probably not allowed at a graphical login). You can also
become root in a terminal on your desktop by using the command
su - and entering the password for root.
What will not change is your user password, ie the one you use to log in with and the one you use when you use
The difference between "becoming root" and using
sudo is that if you are going to perform a series of commands that use root priviledges, you can become root and then run the commands at the prompt (the prompt will normally change from a
$ to a
# and may also change colour to show that you are now the root user) whereas if you stay as yourself and use
sudo, each command you enter needs to be preceeded by