A daemon is a process that removes itself from the terminal and disconnected, executes, in the background, waiting for requests and responding to them. It may also be described as the background process that does not related to a terminal session. Many system functions are commonly acted by daemons, adding the sendmail daemon, which operate mail, and the NNTP daemon, which operate USENET news. Many other daemons can exist. Some of the most usual daemons are:

• init: Takes over the basic executing of the machine when the kernel has completed the boot program.
• inetd: Responsible for starting network application services that do not have their own standalone daemons.
• cron: Responsible for executing repetitive tasks on a usual schedule.