Only root user has write access under this directory .
# ls -ld /
drwxr-xr-x 36 root root 4096 Jul 23 13:06 /
many users assume it as root's home directory .
roots home directory is /root and not /.
# echo $HOME
This directory contains the most commonly used commands.
The commands in this directory are used by almost all the users in the linux system.
For example .
# which cp
# which grep
# which mv
# which ls
Note : Which used to find the location of command.
In above example cp,grep,mv and ls commands are stored in /bin directory.
Therefore they are called as user binaries.
This directory contains commands that are normally used by system administrators.
Commands like iptables (Used to modify firewall setting),ifconfig (Used to change ip address of machine) , fdisk (Used to make filesystem level changes) and many more.....
# which iptables
# which ifconfig
# which fdisk
Therefore they are called as System binaries.
A good article on linux directory structure can be found here.