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.
 Common daemons
- 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.
 Daemon maintainance
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.