Daemons (or Services) are resident programs loaded at startup. Each runlevel corresponds to a list of daemons to start (1 to 5) or to stop (6 or 0).
Other programs than daemons can also be started from the start of the machine, with the same mechanism.
The runlevels, or “runlevels”, correspond to the services that will be launched at the start of the machine. In general (but not all distributions use the same numbering), you can have the following levels of execution:
0: Stops the machine.
1: simple user mode (or single, or failsafe). This mode is useful for troubleshooting a planted machine.
3: console mode. The services are started, but the X server is not activated (and the services it depends on).
4 (Slackware) or 5 (Mandrake, RedHat ..). : Graphical mode: the X server and the services on which it depends are launched.
6: Restart the machine.
The execution level is determined (in order) either:
During the boot: If you specify a level on the kernel command line (for example, at the LILO prompt, type “linux 1”),
In the / etc / inittab file, where the default runlevel is set,
By the command init <runlevel> which allows to change runlevel during operation.
Here is the List of services
These services are launched by scripts located in /etc/init.d (or in /etc/rc.d/init.d, which are often the same file, one being a link to the other). Script contains a description that lets you know what each daemon does at the beginning of the script.
The /etc/rc.d/ directory also contains directories named rcX.d (with X runlevel number). Each of these directories contains a link to the scripts located in init.d.