Ars Technica compares it to an enterprise feature called Automated Tiered Storage.
In a caching solution, like Intel's, files live on the hard disk drive and are temporarily mirrored to the SSD cache as needed. In an enterprise auto-tiering situation, and with Fusion Drive, the data is actually moved from one tier to another, rather than only being temporarily cached there.
The Mac Observer reports that there are two separate drives that appear as one logical partition. As a result, if your Hard Drive fails, it could be replaced with a 3rd party drive and reconfigured as a Fusion Drive.
Meanwhile, they note that all writes take place on the SSD drive, and are later moved to the mechanical drive if needed, resulting in faster initial writes. The Fusion will be available for the new iMac and new Mac mini models announced today.