While introducing the new ridiculously thin iMac, Apple also revealed its new Fusion Drive. What is it? It's a new storage system that supposedly combines the best of SSD and HDD. Basically, the speed of a SSD with the storage space of a big spinning HDD.
The Fusion Drive is made of a 128GB SSD and a 1TB or 3TB HDD that's fused into a single volume through software. How does it work? Apple says it can figure out which apps and stuff you use the most and shift those apps onto the SSD while keeping other less frequently used apps on the HDD. Basically, the OS and your core apps are zippy fast on the SSD while documents are on the HDD.
During the keynote, Apple demonstrated how Fusion Drive would work if you're a heavy Aperture user. Aperture would be moved to the SSD storage side and would perform nearly 3.5x faster (over the old spinning disk of HDD) and nearly matches traditional, SSD-only storage.
Of course, hybrid drives and smart caching and things like that aren't exactly brand new. Fusion Drive is just Apple branding something that already exists.