ReadyBoost, makes PCs running Windows Vista more responsive by using flash memory on a USB drive (USB 2.0 only), SD card, Compact Flash, or other form of flash memory, in order to boost system performance. When such a device is plugged in, the Windows Autoplay dialog offers an additional option to use it to speed up the system; an additional "ReadyBoost" tab is added to the drive's properties dialog where the amount of space to be used can be configured.
Read more about this topic: Windows Vista I/O Technologies