From Fedora Project Wiki
Definitions
Service: A process or task executed and controlled by the init system (e.g. systemd).
Traditional Service: A service which is explicitly started or stopped, either by the init system at boot or manually by a superuser. In systemd, one of several types of service controlled by a .service file.
Socket-activated Service: A service which is waiting for traffic across a socket before activating. In systemd, controlled by a .socket</cod> file.