PC users who watched Apple's Mac announcements last week may have recognized the concept of the Fusion Drive--it's something Intel has been supporting since 2010 with its Z-series motherboards for Sandy Bridge and Ivy Bridge processors. Intel Smart Response Technology essentially uses a solid state drive to cache data for a larger hard drive, so you see a large speed boost compared to using a hard disk alone, but have much more storage space than a small SSD can provide.
News of an Apple patented hybrid drive hit last year, and now it's a reality. Thanks to Ars Technica, we know that Apple's Fusion Drive doesn't work exactly like Intel SRT, even if the technology sounds similar. They studied the posts of a blogger trying (and succeeding) to mimic the Fusion Drive with an SSD, a USB HDD and a little command line trickery. He combined the two drives into one logical volume and saw the operating system write files to the SSD until it was full, and then begin writing to the HDD.
After the drives were full, he read only specific directories over and over. Eventually, after giving the system a chance to go idle, the OS transferred the most-read directories to the SSD, dramatically boosting access times for the next test. He also determined the Fusion Drive is block-based and not file-based, which is advantageous for working with large, regularly changing files and for dealing with the limited storage space of solid state drives.
Even if you don't purchase a Fusion Drive from Apple, you can set one up for yourself with a spare SSD and hard drive. It's noteworthy, though, that Fusion doesn't fit the same use case as Intel SRT. Intel's SSD caching solution is designed for small solid state drives, from 8GB - 64GB--drives too small to comfortably install Windows on. Apple is targeting larger drives, like the standard 128GB size--drives that are already big enough to comfortably install your OS on.
Fusion is a very Apple technology. It's designed to simplify storage management. Yes, it will make your computer faster, but it's more about removing the user effort that goes into choosing what gets stored on the SSD and what gets relegated to the slower, larger volume.
If you want to try setting up your own fusion drive, check out blogger jollyjinx's post here.