The hybrid drives have 8gigabytes(4 on the older ones) of flash to hold often used data, but anything that does not stay in the cache will run at normal hard drive speeds.
Those are great for someone who wants/needs the extra space.
How the SSD on the other hand will be FAST over the full 256gigabytes of flash(you should try NOT to fill it to the max for the sake of its own performance and self maintenance abilities.).
So yeah, just what rolli59 said with a bit more explanation.
SSD = FAST and lower space
SSHDD = Lots of space and decent speed for often performed tacks
Just to give you an idea of speed among drives here is an image.
Also note while not shown here, SSD's have VERY low access times making them feel(and perform) even faster.
The ram drive was just in the image. If you have lots of ram, you can use one, but it has to be loaded on startup and saved on shutdown(slower). And if you crash the latest save will be missed. It is good for things like web browser cache or small writes you want off the SSD. It is in no way required.