I assume that the OS is loaded on the SSD, and various other programs/data are on the HDD. But what if the SSD is not large enough to store the OS (many SSD drives in these configurations are 24GB, and I believe Windows can be larger than that)? In other words, can the OS span the drives? If I buy one of these, out of the box, what should I expect to be on the SSD and what on the HDD?
Hybrid drives are the combination of a traditional magnetic disk drive and a solid state disk, however, they are not two separate drives. Hybrid drives store everything on the magnetic drive. Using its own algorithm it caches frequently used data to the SSD. This gives you the storage space of the much larger magnetic drives, with (some of) the increased speed of solid state drives. The OS does not see the disk as two drives, but simply as one drive.
This is a good article to read.