In disk storage, data is recorded on planar, round and rotating surfaces (disks, discs, or platters). A disk drive is a peripheral device of a computer system, connected by some communication medium to a disk controller. The disk controller is a chip, typically connected to the CPU of the computer by the internal communication bus. Main implementations are hard disks, floppy disks and optical discs, of which the first is the usual interpretation.
Recently, Solid State Disks have been introduced; though the term “disc” is a misnomer for these devices, as internally they consist of NAND Flash memory chips. Similarly, the term RAM Disk is used for a storage device consisting of volatile DRAM memory. Both offer the same data storage abstraction as a hard disk at the operating system level, though their price, size, performance and persistence characteristics are very different from a hard disk.