Most RAID-class NASes have supported iSCSI for some time and iSCSI (Internet Small Computer System Interface) has been around for awhile and was developed as a SAN (Storage Area Network) protocol.
You can think of iSCSI as a way to provide computers with the illusion of large volumes of direct-attached storage, while the storage actually sits in a NAS or usually larger storage farm somewhere on the network. The QNAP diagram below illustrates the concept.
smallnetbuilder has a nice explanation of how iSCSI performance compares to SMB performance. You will be surprised about the results: NAS Too Slow? Try iSCSI – Setup-more, Features, Performance.