From LQWiki

A daemon is a program or process that runs in the background on a Linux system in order to perform a system-wide service, such as an HTTP or FTP server. It is usually written to consume minimal resources, until explicitly invoked to perform a task (for example an HTTP request is received).

The term comes from Maxwell's Demon, later incorrectly retronymed as "Disk And Execution MONitor’.

Daemons often run with root privileges, although some are designed to make use of a specialised user account for that daemon.

The Microsoft Windows equivalent of daemons are called "services".

The idea of a daemon is that the perpetrator of the condition need not be aware that a daemon is lurking (though often a program will commit an action only because it knows that it will implicitly invoke a daemon). For example, under ITS, writing a file on the LPT spooler's directory would invoke the spooling daemon, which would then print the file. The advantage is that programs wanting (in this example) files printed need neither compete for access to nor understand any idiosyncrasies of the LPT. They simply enter their implicit requests and let the daemon decide what to do with them. Daemons are usually spawned automatically by the system, and may either live forever or be regenerated at intervals.

Daemon and demon are often used interchangeably, but seem to have distinct connotations. The term daemon was introduced to computing by CTSS people (who pronounced it /dee´mon/) and used it to refer to what ITS called a dragon; the prototype was a program called DAEMON that automatically made tape backups of the file system.

This article is based, in whole or in part, on entry or entries in the Jargon File.

Common daemons

Historically daemons end in the letter "d", although this is not always the case (e.g. portmapper and kudzu).

• httpd, the daemon for the Apache web server.
• ftpd, an FTP server.
• ntpd, the Network Time Protocol (NTP) service.
• crond, providing the cron regular scheduling service.
• atd, providing the at one-off scheduling service.
• cupsd, the daemon for the Common Unix Printing System.
• xinetd, a generalised server daemon which controls various network servers.