I am learning command line from a book called "Linux Command Line and Shell Scripting Bible, Second Edition." The book states this:
Some Linux implementations contain a table of processes to start automatically on bootup. On Linux systems, this table is usually located in the special file /etc/inittabs.
Other systems (such as the popular Ubuntu Linux distribution) utilize the /etc/init.d folder, which contains scripts for starting and stopping individual applications at boot time. The scripts are started via entries under the /etc/rcX.d folders, where X is a run level.
Probably because I am new to linux, I did not understand what the second paragraph quoted meant. Can someone explain the same in a much plainer language?