□ su(substitute user)
○ This linux command allows a user to log in as another user Without log out
○ "su" without parameter is same as "su root"(=Login as root)
○ Account password to login is required

□ sudo(substitute user do)
○ This linux command allows a user to execute as another user's authority Without log out
○ "sudo" without parameter is same as "sudo -u root"(=Execute as root's authority)
○ Current login account's password is required

□ Difference between "su" and "sudo su"
○ su : system admin(=root) account's password is required
○ sudo su : Current login account's password is required......

It also posted at : http://cysecguide.blogspot.com/2016/09/difference-between-sudo-and-su.html