/ : this is root directory
/bin : commands in this dir are all system installed user commands
/sbin: commands in this dir are all system installed super user commands
/usr/bin: user commands for applications
/usr/sbin: super user commands for applications
/usr/X11R6/bin : X application user commands
/usr/X11R6/sbin: X application super user commands
normally you can't execute super user command like visudoer, /usr/local/sbin/apachectr, etc when you using a user login.
