Unlike memory, which is fast, small, and volatile (looses its contents when the computer is turned off,) a typical storage device is slow, big, and non-volatile. Programs and important data are kept in storage.
Most common type of storage device is a hard disk drive (also called a hard disk, disk drive, or HDD). An HDD consists of a disk coated with a magnetic material. The disk spins very rapidly under read-write heads that can move back and forth over any track of the disk.
In essence, a disk combines the ideas of a record and a tape. Like a tape, information is stored magnetically; like a record, information is stored in concentric tracks.
Having moving parts slows the HDD down compared to memory. Storing information magnetically is what makes in non-volatile.
Disks can have huge capacities compared to memory.
Other types of storage devices include compact discs or CDs, DVDs, and USB flash drives.