Systemd
Service and timer files go in /lib/systemd/system/
They have to be activated by being enabled and started, for example
systemctl daemon-reload systemctl enable backup.service systemctl start backup.service
Service types
Type= one of these:
oneshot - "simple" but has to exit before follow up services run. It can run several commands stacked in ExecStart=
simple - (default) run one time when started; can be a daemon or just a simple run of a script
idle - like simple, but wait until other services have finished running
forking - use if starting a program that does the traditional fork-and-exit startup; otherwise you want "simple" most of the time
dbus - for services that will run on dbus
notify - your program should send a 'notify' to tell follow ups they can run
References
Most helpful for me: https://www.freedesktop.org/wiki/Software/systemd/
Generic information for Debian: https://wiki.debian.org/systemd