What is the difference between 'su' and 'su -'

firstly

su is the same as su root

su - is the same as su - root

su or su username

becomes superuser root (if no parameters are specified) or the specified user without changing the current environment variables such as PATH and PWD. I assumes the privilege without changing the current environment

su - or su - username

becomes superuser root (if no parameters are specified) or the specified user as if you have logged out and logged in again as that user. It changes to the new environment and privilege of the user e.g. PATH and PWD will that of the user as well as privileges

Reference

[1] http://www.linuxforums.org/forum/red-hat-fedora-linux/71481-what-difference-between-su-su.html