SSD-Caching is a description for a technology, which combines the advantages of fast SSDs and cheaper, traditional HDDs. The solution for SSD-Caching from Intel is called Rapid Start.
SSD-Caching is either implemented hard- or software-based. Frequent used data and software or parts of them, will be copied from the hard disk (HDD) to the SSD. If they are needed for the next time, they will be loaded from the SSD which lead to considerable increased performance advantages and a smoother user experience.
Ultrabooks with SSD-Caching will be faster after every usage, because the Caching determine on the basis of the user work utilization, which data and software is frequently used. The capacity of Cache-SSDs, which will be mostly used for saving the hibernation file when entering the S4 idle mode too, is between 16- and 32GB. If a software has been frequently started, but then it is not used anymore, it will be deleted from the dynamic SSD-Cache and normally loaded from the HDD.
Compared to a normal SSD (e.g. with the system partition on it), SSD-Caching cannot provide the same performance. In addition it cannot be used for applications (e.g. modern games), which are loading large files. Therefore the Cache-SSDs are obviously too small.