At the iPad mini event, Apple announced the Fusion Drive. It’s an option for users who want to use a drive with the speed of Flash, but the capacity of an HDD.
Here’s how Apple describes it:
Fusion Drive is a new storage option for Mac mini (Late 2012) and iMac (Late 2012) computers that combines the performance of Flash storage with the capacity of a hard drive.
Presented as a single volume on your Mac, Fusion Drive automatically and dynamically moves frequently used files to Flash storage for quicker access, while infrequently used items move to the hard disk. As a result you’ll enjoy shorter startup times, and as the system learns how you work you’ll see faster application launches and quicker file access. Fusion Drive manages all this automatically in the background.