If you use 'su', you need to enter root's passwd, not your own.
NB: this logs you in as root, BUT with your original env. To get root's env (eg $PATH etc), use
You would use sudo with your own passwd.
Its perfectly valid to enable some users to access a limited set of privileged cmds via sudo/sudoers; that's what it was invented for.
What's peculiar to Ubuntu (& derivatives I believe) is that the root acct is disabled and the first user acct created is automatically given access to ALL cmds via sudo; NOT what it was created for...