During their event on October 23, 2012, Apple announced the Fusion Drive as a new form of data storage available for the iMacs and Mac minis. Fusion Drives are composed of one part hard disk and one part solid-state drive. Combining the two drives gives the computer and the user the best features of both storage technologies.
The hard disk provides a large capacity of storage while the solid-state provides the fastest speeds; all at a fraction of the cost of a fully solid-state drive of the same capacity.
The software of the operating system also plays a role in utilizing the most speed out of your Fusion Drive. The OS recognizes frequently used data, files, and applications and stores them in the solid state portion of the Fusion drive.
The items that are accessed less frequently are stored in the larger capacity RPM hard disk drive. During times where the machine is in idle mode, the data is moved to and from the solid state portion so users only notice the fastest speeds from their machines.