What is this long post about?
iSCSI is standard for accessing block devices (e.g. disks) over network, just as if they were local SCSI devices. That's similar to AoE and FCoE, although the latter two are good for the LAN only, while iSCSI is over IP thus is good on WAN. This article would focus iSCSI but could be used as a base for doing similar things with AoE and FCoE.
So, iSCSI in the simplest configuration, allows us to mount and manage a data disk that is physically connected to a remote computer (the "server", aka target) from our own computer (client, aka initiator) .
On this post I'll discuss the deep details of the more advanced stage: having the root (also boot) disk on a remote computer, so client could remotely boot from it. Surprisingly it can be done even with relatively old hardware.