Fusion drive is Apple's implementation of hybrid drive. It is simply combining a Hard Disk Drive and an SSD (Solid State Disk) with the space of both drives combined. Creating a Fusion Drive Volume is just like creating a Software RAID Volume.
The operating system automatically manages the contents of the fusion drive so the most frequently accessed files, applications, documents, photos and other data are stored on the faster flash storage (SSD), while the infrequently used items move to or stay on the hard disk drive. Simply, 1TB HDD + 128GB SSD Fusion Drive Volume gives you 1.1TB SSD speed drive in usage.
You can easily create your Fusion drive by using terminal in 2 steps. We will also create a GUI application to make it user friendly (also for RAID operations for El Capitan).