A daemon is a program that runs as a background process and does some specific task. They are usually started at boot or manually (re)started by running the daemons startup script which can be found in /etc/init.d/ on most variants of the The GNU Operating System.
Most daemons allow you to send them signals such as KILL (commonly known as -9) using the command "kill". kill doens't have to kill the process, it has many other uses. For example, the -HUP command typically causes a daemon to reload it's configuration file. See the kill manual for a complete signals list.
- portman, the port to RPC program number mapper.
- cups-polld, which waits for applications to send it a printing job and then hands it off to the printer or printer server.
- dbus-daemon, the freedesktop message bus daemon
- and on and on. Run ps aux to get a list of all the processes running on a system.
CentOS, RHEL and Fedora use the command chkconfig to control which daemons are started at boot. For example, chkconfig httpd on makes Apache start at boot.
Gentoo uses the command rc-update, for example, rc-update add tor default makes the Tor daemon start at boot.