according to Wikipedia…
A solid-state drive (SSD) is a data storage device that uses solid-state memory to store persistent data. Unlike flash-based memory cards, an SSD emulates a hard disk drive, thus easily replacing it in most applications. An SSD using SRAM or DRAM (instead of flash memory) is often called a RAM-drive.
The original usage of the term solid-state (from solid-state physics) refers to the use of semiconductor devices rather than electron tubes, but has in this context been adopted to distinguish solid-state electronics from electromechanical devices as well. With no moving parts, solid-state drives are inherently less fragile than harddisks and therefore also silent (unless a cooling fan is used); as there are no mechanical delays, they usually enjoy low access time and latency.
SSDs have begun to appear in laptops, although they are at present substantially more expensive per unit of capacity than hard drives.