In a nutshell, Fusion Drive combines TWO drives, a solid state
drive (SSD) and a mechanical drive. Apple’s initial incarnation
puts a 128GB SSD in with either a 1TB or a 3TB mechanical drive.
This is not a simple RAID, however, as 100% of the “magic” is done
within OS X itself. What Fusion Drive does is it watches what
files and applications you run most regularly and it moves them
to the SSD. It will also take stuff that you aren’t using all that
often and moves it to the mechanical drive.