SuperFetch is a technology that pre-loads commonly used applications into memory to reduce their load times. It is based on the "prefetcher" function in Windows XP. SuperFetch attempts to load commonly used libraries and application components into memory before they are required. It does so by continually analyzing application behavior and usage patterns, e.g. what applications are typically used in the morning after logon. The cache memory is marked with low priority, meaning that if another process needs the memory, it will be given up.
By default the necessary files are loaded into main memory, but using a feature called ReadyBoost, Windows Vista and Windows 7 can use alternative storage such as USB flash drives, thereby freeing up main memory. Although hard disks usually have higher sequential data transfer rates, flash drives can be faster for small files or non-sequential I/O because of their short random seek times.
Read more about this topic: Windows Vista I/O Technologies