systemd replaces the SysV init system: services are managed with systemctl,
and runlevels map to targets.
1systemctl status <unit> # status of a service.
2systemctl start|stop|restart <unit> # run / stop / restart.
3systemctl enable|disable <unit> # start at boot (or not).
4systemctl isolate multi-user.target # equivalent of runlevel 3.
5systemctl set-default multi-user.target # change the default target.
6systemctl get-default
See the Runlevels & Shutdown page for the classic runlevel table.