initrd -- what exactly is it?

[Matt Price <matt.price@utoronto.ca>]

hi there,

can someone help me figure out what exactly initrd is, and why
kernels use it?  I have looked through the docs, and I understand that
it's thefile used for an initial ramdisk in some cases, but I don't
understand why it would be used in some cases and not in others.  So
for inst ance, the demudi kernel I just installed seems to demand the
use of initrd, and I take it GRUB needs an anitrd argument to load the
kernel.  But why don't my own self-compiled kernels require an initrd
argument at boot (nor have an initrd file anywhere in /boot, as far as
I can tell)?  

thanks for the (continuing) assistance.  
Matt