[_vOv_]

It's called PXE boot.

You need a server to host the ISO (via http/ftp/tftp or whatever you prefer) and a DHCP server that will distribute the ISO URI to the client.

Configure the client to boot from the network in the bios and put it in the same LAN as the dhcp server.

That's the gist of it.

[LeoPanthera]

Raspberry Pi's don't use PXE boot, they have their own proprietary system. The kernel is fetched by TFTP, along with a config text file which species an NFS location to mount as root. Boot then continues from there.