Command line: systemctl

The go-to command to manage units in a systemd-ful system.

For full details, you can view the systemctl.1 manual page.

Synopsis

systemctl [OPTIONS...] COMMAND [COMMAND_ARGS...] [COMMAND_OPTIONS...]

--user runs the binary in user mode with the user-specific options and load-path. See systemd at user-level for more details.
--help is an obvious one. It prints a detailed help section of the command.
--host controls a remote system. Meaning, you can manage services and timers remotely. Will not work on certain subcommands such as edit.
--container is similar to --host but operates on a local container.
--with-dependencies will show the output of the unit specified along with its dependencies. This is only effective on certain subcommands such as status, list-units, cat, and list-unit-files.
-p PROPERTIES shows only the listed properties. Very useful for filtering the properties for systemctl show subcommand. Take note PROPERTIES can be a list of properties to show delimited with a comma.
-P PROPERTIES shows the value of the listed properies. Similar to -p, PROPERTIES can be a comma-delimited list of properties to be shown.

show PATTERN shows the properties of the matched units. It can also show with multiple units with a helpful label and proper formatting (which is very nice). This is the go-to subcommand for debugging a service.
enable UNIT and disable UNIT enables and disables units. It mainly adds the unit activation at startup.
- --now starts/stops the unit. It is the equivalent of systemctl {enable,disable} UNIT && systemctl {start,stop} UNIT. Pretty handy if you intend to add the unit at startup and want to use it now.
start UNIT and stop UNIT starts and stops the unit, respectively. Though, this doesn't activate
restart UNIT restarts the given unit. This also reloads with the modified changes of the unit.
In some cases where restarting the unit isn't enough, daemon-reload will reload the daemon with the modified units. systemd does not have reload-at-change for its unit. That is, if a unit is modified, systemd will not run the modified version until reloaded.
show-environment shows the environment variables of the system.
list-units [PATTERN...] is self-explanatory ;p.
is-active PATTERN... prints the status of the matched unit and exits successfully if it's active.
cat PATTERN... shows contents of the given units in a neat formatted page.