Hybrid Storage melds solid State Drives or flash memory with hard disk drives to deliver a superior combination of $/GB and $/IOP. Central to any hybrid storage system in an SSD caching architecture that places hot data in SSD and leaves data at rest on less expensive hard disk drives. Dave Floyer has written an article on the benefits of hybrid storage over traditional storage in being able to provide more performance at a lower cost: Hybrid Storage Poised to Disrupt Traditional Disk Arrays.

Hybrid Arrays can be focussed on specific workloads or provide broad consolidation of SAN and NAS. Because of the nature of the hybrid architecture and the use of a thin layer of solid state to drive performance the more applications you consolidate behind a single flash/SSD Cache the more efficiency you get from the flash/SSD resources.

Many vendors have added SSD or flash as a cache to traditional architectures but true hybrid arrays use SSD/flash specifically as a cache in a purpose built architecture.

Also see Wikibon's The Evolution of Flash Memory Infographic.