Fusion Drive is a Apple technology for make a logic unit like RAID 0 combining a traditional HDD and a SSD. You can the advantage of both: The system distribute the information that you use with more frequently in the SDD because you need feel that works fast, and also have too storage.
But the very nice of this is that is transparency for you transparency for you.
If you are interested in make a Fusion Drive, you only need the hardware, an backup (Carbon Copy Cloner is a very nice for this) and typical terminal commands create the new logic unit.
This is a nice guide about the process.
Also remember use Trim Enabler or similar software to activate TRIM cache system for your SSD.
