Linux systems provide a variety of system services (process management, login, syslog, cron, etc.) and network services (remote login, email, printers, web hosting, data storage, file transfer, DNS, DHCP, and more).
Technically, a service is a daemon — a process or group of processes running in the background, waiting to respond to client requests.
Modern Linux distributions use systemd as the default system and service manager. With the systemctl command, you can easily list, start, stop, restart, enable, or disable services.