This is the actual kernel itself
This is a location routine to assign various file and devices on bootup. This is created when the kernel is compiled
Many modern linux distros use a modular kernel, which allows modules to be added without requiring that the kernel be rebuilt. If your root filesystem is on a device whose drive is a module (as is frequently of SCSI disks), you can use the initrd facility , which provides a two stage boot process, to first set up a temporary root filesystem in a RAM disk containing the modules you need to add and then load the modules and mount the real root filesystem. The RAM disk containing the temporary filesystem is the special device file /dev/initrd. It is not always necessary to use an initrd, in fact, I don't on Slackware, but do in Fedora.
This is a file containing the information on how the kernel was compiled, what is enabled and what is available as a module.
Hope this helps