Bootloader - The bootloader, Arlo does for EPOC what Loadlin does for DOS. It releases the memory from EPOC, places the kernel and initrd in the appropriate places in memory, and then boots the Linux kernel.
Kernel - The kernel has patches applied to it, which makes it compile for ARM based systems as well as having a few more patches for the Psion Hardware. The kernel port to the Series 5 is basically finished now.
Initrd - This is a virtual filing system image, which contains a miniature version of Linux. These can range in size but the more RAM used for the RAMdisk will leave less for the operating system. The kernel loads these images as the root partition.