BOOT PROCESS VERIFICATION

terminal

Hi Techs,

Been trying to understand the boot proceess and am not yet able to drive it home. My understanding so far is
1. POST
2. LOADS OS( IO.SYS-MSDOS.SYS)
3. CONFIG.SYS
4. COMMAND.COM
5. AUTOEXEC.BAT

I completely understand steps 1,3,4,5.
As for step 2, what are these files, what do the contain, in what order are the loaded, does it matter in the order they are loaded and finally there importance or relevance to different OSs.

It's also my understanding that autoexec.bat and config.sys are only required in Dos and Win 3.1. My question is how do the newer OSs make up for this?

Kindly help me with this issue in the simplest kind of terms. (Idiots Guide)
thanks