iSCSI carries SCSI commands over TCP/IP. You get block storage without Fibre Channel hardware.
How it works:
Server runs iSCSI initiator
Initiator connects to iSCSI target
SCSI commands encapsulate in TCP
Target presents LUNs as local disks
Advantages: Uses existing Ethernet. Lower cost. Familiar networking.
Considerations: TCP adds some latency. Shares bandwidth with other traffic. Use jumbo frames and dedicated VLANs.
iSCSI approaches FC performance with or Gbps Ethernet.