systemd is an init system used in Linux distributions to bootstrap the user space and manage all processes subsequently, instead of the UNIX System V or Berkeley Software Distribution (BSD) init systems. [...] One of systemd's main goals is to unify basic Linux configurations and service behaviors across all distributions. (Wikipedia)
Usage for units
A unit can be service, mount point, device or socket.
Show status and running units
(1) List running units and (2) show system status
systemctl list-unit-files systemctl status
Start, stop, restart and reload
reload reloads the configuration of the unit.
systemctl start <unit> systemctl stop <unit> systemctl restart <unit> systemctl reload <unit>
Enable and disable bootup
(1) Enable a unit to be started on bootup and (2) disable it.
systemctl enable [--now] <unit> systemctl disable <unit>