What is the difference between `su` and `su -` in CentOS?

Adding the - option affects your environment behavior. For all practical purposes, the environment is completely reset. In general, you likely want to use su - instead of plain su. From the man page:

 -l      Simulate a full login.  The environment is discarded except for HOME, SHELL, PATH, TERM, and USER.  HOME and SHELL are modified as above.
         USER is set to the target login.  PATH is set to ``/bin:/usr/bin''.  TERM is imported from your current environment.  The invoked shell is
         the target login's, and su will change directory to the target login's home directory.

 -       (no letter) The same as -l.

su - invokes a login shell, which among other things, ensures that root's .bashrc and other shell startup scripts are run, just like as if you'd logged in directly as root via console or SSH. root's profile usually sets your path to include /sbin which is where ifconfig generally lives.