It's used to store frequently used data. If a read request finds data is available in the SSD, the drive will use that - greatly increasing performance.
The drive controller independently determines what data to store in the SSD. The OS doesn't know whether the data is read from the SSD or HD, and has no idea of what data is written to the SSD.
As far as the OS is concerned, there is a single 500GB drive. The SSD essentially mirrors frequently accessed data; like an additional large non-volatile cache.
You can read more about it at Seagate's site. Also, search for reviews at the major tech sites.