iSCSI

The two big differences between NAS and Fibre Channel SAN are the wires and the protocols. In terms of wires, NAS runs on Ethernet, and FC-SAN runs on Fibre Channel. The protocols are also different. NAS communicates at the file level,FC-SAN communicates at the block level.
If you think the protocol is more important, then iSCSI is like SAN; if the wire is more important, then iSCSI is like NAS.With FC-SAN, a filesystem like UFS, VxFS or ZFS runs on the host and converts file requests into the block requests that are sent over the wire. With NAS, the host sends file requests over the wire, so a file system must run in the storage system.From a technical perspective, iSCSI looks more like SAN, but in terms of business issues, it looks more like NAS.
People keep comparing iSCSI with Fibre Channel. They point out that iSCSI is slower, and less mature, and less reliable, and less quality of service, and more variability in packet latency.iSCSI is about enabling networked storage in areas where Fiber Channel is completely impractical.Networked Storage has almost completely replaced DAS (Direct Attached Storage) at the high end.
The two key drivers for iSCSI are cost and simplicity.Every computer these days is networked, so iSCSI adds zero cost.iSCSI is simple,No new hardware to add and not much to configure.I myself has finished configuring iSCSI it is really simple.

Dell's View on iSCSI:
http://direct2dell.com/one2one/archive/2007/04/17/12165.aspx

helpful blog:
http://blog.scottlowe.org/tags