1. What is root file system used by linux ?
$ cat /etc/fstab
it's the one with a mount point of `/'. See man fstab
2. what is initial ramdisk? Why this is necessary? Can linux bypass using initial ramdisk ?
basically it's using your RAM as a file system for speedup. I don't think it's necessary per se, but it's a Good Thing (tm).
$ find / -name '*initrd*' # or
$ locate initrd
beside this, I have some other questions regarding:
3. What is LILO and what about lilo installer? whats the difference between them?
gnu/LInux LOader; a boot loader that's simpler and less powerful than grub, and thus less error-prone
(don't get me wrong, I haven't been bitten by grub or anything--but the KISS principle *always* applies)