The initrd file contains only those modules necessary to boot and mount your / filesystem (and a few other things to help out). Typically it's filesystem modules, scsi controller modules, or raid (md) modules.

When the computer boots it just starts reading the hard drive at a certain point and executing the code it finds. Your boot loader makes sure this is your kernel. The initrd file contains all of the modules the kernel needs to get access to the rest of your system, because the kernel was't started knowing about your disk controllers and filesystems.

At a certain point the system does the "pivot_root" process which is where the filesystems are actually mounted and programs other than the kernel can run, like init.

Make a copy of your initrd file. Add a .gz to the end of the filename. gzip -d copyofinitrd.gz; mount -t ext2 -o loop copyofinitrd /mnt; cd /mnt; ls

And you can look at what's inside of the initrd file.

--Nathan