What's initrd image?
 

I've realized linux distributions comes with a initrd file (e.g initrd.img-2.4.23), and i would like to know what's this file.

Is it necessary? Must i create another one when i compile my own kernel?

Thanks in advance.

If you compile your filesystem as a module, you will need at initrd (initial ram disk). If I am right, your kernel cannot mount / directory if the file system is compiled as module which leads to the problem of system actually not booting. Initrd is sort of a work around that problem. Kernels compiled by Redhat and Fedora have ext3 filesystem as a module hence they need initrd.

You can compile your filesystem directly into the kernel and do away with initrd.

google for more info.