Its completely different than a hybrid drive.
Actually, it's a similar idea implemented in reverse.
In old hybrid drives, you have a small amount of SSD space that's used to cache regularly accessed data. Like the OS would boot of it, as would a few of your commonly used programs. Everything else is shunted to the HDD.
With a fusion drive, you have a moderately sized SSD that everything is written to first, and moved over to the HDD when the SSD reaches full capacity.
A hybrid drive sounds like it'd work better in practice, but I haven't ever heard many good things about it. The fusion drive, which seems like a sloppier implementation to me, seems to work beautifully according to everything I've read about it. It gives you the SSD boost, without having to worry about managing two disks.
The only problem I could see with it is if you want to run something off the SSD, but it's stuck at the back of the drive. How would you give certain programs access without having to uninstall a bunch of stuff?