su - && su - root: the same, login as root, pure login of root, totally new login of root, .bash_profile and .bashrc will be sourced.
su root && su : almost the same. A interactive shell will be open, use the exit will jump to the former users‘ shell. Only .bashrc will be sourced.
su && su -: su will not change the execution environment while su - will change the execution environment.
su && sudo: sudo is one strict kind of su. Refer.
原文：http://www.cnblogs.com/flowerencedew/p/4494337.html