Mik is absolutely correct.
With su, by itself, you get root permissions, but with su - you get the root path environment also. The difference being that without the path environment, finding and executing root permissioned executables becomes more of a problem.
What I always do when I need root access at the CLI when I'm my "normal" user:
<Respond to password prompt>
<Do what I need to do>
And I'm back to my "normal"user.