System files in Operating system

[Sadanand]

Hi all,
what does system files contains in Windows Operating system?
most of the system files have a file ext .sys
what does a .sys files contains?
does it contain object code of some OS code?
does it contain useful information that is used by OS?
Can we explore the content ? if it is just a useful information
rather than object code of OS code.
how exactly they have been used within the system?
Please provide me some links or explain me with an example.

Thanks & Regards,
Sadanand.