vmlinuz it the file name and located into /boot in rhel4, rhel5, this is actually kernel.
initrd is initial ramdisk file that is used to provide some kernel modules while booting or executing kernel.
Kernel, is the just a name, vmlinuz is the kernel, if there is "z" (vmlinuz) in the last, it means kernel is comprised and if there is "x" (vmlinux) it means it is uncompressed kernel.
boot => /boot is the directory where all the booting related stuff's are stored, like stage2 grub bootlaoader, grub configuration file, kernel(vmlinuz), initial ram disk file (initrd)
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