This is intented to duplicate the directory stucture that
would normally start at /

This is the dirs that pxe tells the bootloaders
where to grab the boot loaders from, the kernels,
any initrd, configuration files, etc.

The most interesting directory is the pxelinux
subdirectory. This contains a binary version
of the pxelinux boot loader, a linux kernel,
and a subdir containing a configuration file
for pxelinux.

The boot process works more or less like this:

machine powers up
the network hardware dhcp's for an ip address

This request is answered, but proxied by the PXE
server. The pxe server also adds in a bit of extra
info, like where to grab a network boot loader
or kernel from.

The machine tftp's the bootloader, usually from the
same machine as the PXE server. The bootloader
, in our case pxelinux, should know where
to grab the kernel from. pxelinux can also
grab a third file, a configuration file 
that can include info like kernel paramaters,
vga modes, etc. 

The bootloader then fetches the kernel via tftp,
passing it any extra arguments as specified in the
configuration file, and boots the kernel.

The kernel boots up to the point where it needs its
ip addres, it then sends out a bootp request, which
gets answered by the DHCP server. It sets
it's own ip address to this, and unless told otherwise,
it assumes the dhcp server is also its root NFS server.

The kernel then mounts / from NFS, and continues the boot
more or less as normal at that point.