systemd - ArchWiki
Historically, what systemd calls "service" was named daemon: any program that runs as a "background" process (without a terminal or user interface), commonly waiting for events to occur and offering services. A good example is a web server that waits for a request to deliver a page, or an ssh server waiting for someone trying to log in. While these are full featured applications, there are daemons whose work is not that visible. Daemons are for tasks like writing messages into a log file (e.g. syslog, metalog) or keeping your system time accurate (e.g. ntpd). For more information see daemon(7). The main command used to introspect and control systemd is systemctl. Some of its uses are examining the system state and managing the system and services. See systemctl(1) for more details. Units commonly include, but are not limited to, services (.service), mount points (.mount), devices (.device) and sockets (.socket). When using systemctl, you generally have to specify the complete name of t