Daemon

A background process providing a service, usually started at boot.

A daemon is a long-running background process with no terminal attached: web servers, databases, SSH servers, schedulers. Their names traditionally end in "d": sshd, nginx, crond, systemd.

On modern Linux, daemons are managed as systemd services: you start, stop and enable them with systemctl, and their output lands in the journal rather than a terminal. The word has nothing to do with demons: it comes from Maxwell's demon, a helpful background entity in physics.

See it in practice

$ systemctl list-units --type=service --state=running

Every daemon currently running under systemd, with a one-line description of each.

Where you'll meet it

systemctl

Start, stop, enable and inspect services: the control panel of modern Linux.

journalctl

Query the system journal: by service, time window, priority, and follow live.

Process

A running program, with its own ID, memory and environment.

Errors that come down to this concept: Connection refused

Learn these concepts by typing them: the interactive course builds them up lesson by lesson in a real simulated shell.