What is the command to run System Settings from a terminal?

gnome-control-center

Note: for versions between 14.04 and 17.04, the command is unity-control-center

if you run

gnome-control-center

and get

gnome-control-center: command not found

you can install with

sudo apt-get install gnome-control-center

For Ubuntu Mate the command is mate-control-center.