During today's press event, Apple revealed that both the 2012 Mac mini and the 2012 iMacs would have an option for a Fusion drive. The Fusion drive is a combination of a 128 GB SSD and either a 1 TB or 3 TB standard hard drive.
Combining an SSD with a hard drive isn't that unusual; drive manufacturers have been offering similar systems for a while. What makes Apple's Fusion drive unique is the software it uses.
Apple's Fusion drive uses a separate 128 GB SSD and a 1TB or 3 TB hard drive. The Fusion software combines the two drives so that the OS sees them as a single volume. The really interesting part is that the Fusion software monitors which apps and data you use frequently and then moves them to the SSD section, which ensures that they'll operate at the much faster data transfer rate that an SSD offers. Apps and data you use infrequently are moved to the hard drive section.
Apple says the SSD section will also house the OS, to ensure peak performance.
The Fusion software automatically moves data around and is included in OS X Mountain Lion, although the specific version number wasn't mentioned. It also wasn't clear whether the Fusion software will work for users who supply their own SSD and hard drive or only for Apple-supplied configurations. I suspect the latter, but we should be able to determine the answer to that before long.