As a part of the big iMac announcement today, Apple introduced a new kind of storage for their computers called Fusion Drive. It combines both flash and traditional HDD storage into a single logical drive.
The way it works is that your Mac will automatically move over frequently-used applications or you can choose what options you'd like on the flash portion of your hard drive and which files you want on the regular part of your hard drive. This means that applications that are on flash storage will run a lot quicker than those on the traditional portion of the HDD.
Solid state hard drives are still quite pricey and gets iMac users a few more options when it comes to speeding up their computers as well as having mass storage at a much more reasonable cost.