Glossary
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=runningEvery daemon currently running under systemd, with a one-line description of each.
Where you'll meet it
Start, stop, enable and inspect services — the control panel of modern Linux.
journalctlQuery the system journal: by service, time window, priority — and follow live.
ProcessA running program, with its own ID, memory and environment.