Reference:

[1] https://www.linux.com/news/introduction-services-runlevels-and-rcd-scripts

A Linux service is an application (or set of applications) that runs in the background waiting to be used, or carrying out essential tasks.

In the directory /etc/rc.d, you will find either a set of files named rc.0, rc.1, rc.2, rc.3, rc.4, rc.5, and rc.6, or a set of directories named rc0.d, rc1.d, rc2.d, rc3.d, rc4.d, rc5.d, and rc6.d. You will also find a file named /etc/inittab including info about runlevels.The system uses these files (and/or directories) to control the services to be started.The boot process uses these parameters to identify the default runlevel and the files that will be used by that runlevel. 

About runlevel, you might assume that this refers to different levels that the system goes through during a boot up. Instead, think of the runlevel as the point at which the system is entered. Runlevel 1 is the most basic configuration (simple single user access using an text interface), while runlevel 5 is the most advanced (multi-user, networking, and a GUI front end). Runlevels 0 and 6 are used for halting and rebooting the system. The following table shows a generic list of configurations (and some examples of different distros) taken from Linux - The Complete Reference (R.Peterson, Osbourne/McGraw-Hill).

0:关机 
1:单用户模式 
2:无网络支持的多用户模式 
3:有网络支持的多用户模式 
4:保留，未使用 
5:有网络支持有X-Window支持的多用户模式 
6:重新引导系统，即重启 
Linux 系统处于正常状态时，服务器主机开机(或重新启动)后，能够由系统引导器程序自动引导 Linux 系统启动到多用户模式，并提供正常的网络服务。如果系统管理员需要进行系统维护或系统出现启动异常时，就需要进入单用户模式或修复模式对系统进行管理了。使用单用户模式有一个前提，就是您的系统引导器(grub)能正常工作，否则要进行系统维护就要使用修复模式。特注:进入单用户模式，没有开启网络服务，不支持远程连接 
Linux 系统中不同的运行级别(Run Level)代表了系统的不同运行状态，例如 Linux 服务器正常运行时处于运行级别3，是能够提供网络服务的多用户模式;而运行级别 1 只允许管理员通过服务器主机的单一控制台进行操作，即“单用户模式”。

Linux Boot Process

When you boot your computer, the first thing that it will do is load the bootloader -- either GRUB or LILO in most cases. The bootloader will then load the Linux kernel -- the core operating system. Next, a process called init starts. This process reads the file /etc/inittab to establish the runlevel to use. The runlevel is the start mode for the computer.

Once init knows the runlevel it will look for the appropriate files or directories as defined in /etc/initab.

Init will then either run the script files defined by /etc/initab, or run the script files in the directories defined by /etc/initab (depending on the set up of your system).

Finally, init will present you with the logon mode that you‘ve selected.

Linux services, runlevels, and rc.d scripts