What is a systemd unit?

Systemd units define resources that can be used by the system. Examples of these units are a service, path, socket, and timer. Each unit type has its own basic set of properties that then individually can be configured. Unit types can be recognized by their file extension. A service will use the ‘.service’ extension, making it easy to recognize. Units are usually managed with the systemctl command.

See systemd unit types and their purpose for a full overview of the units.