Storage devices retain data even when the computer is turned off.

These are the main types of mass storage:

- Floppy disks : Relatively slow and have a small capacity, but they are portable, inexpensive, and universal.
- Hard disks : Very fast and with more capacity than floppy disks, but also more expensive. Some hard disk systems are portable (removable cartridges), but most are not.
- Optical disks : Unlike floppy and hard disks, which use electromagnetism to encode data, optical disk systems use a laser to read and write data. Optical disks have very large storage capacity, but they are not as fast as hard disks. In addition, the inexpensive optical disk drives are read-only. Read/write varieties are expensive.
- Tapes : Relatively inexpensive and can have very large storage capacities, but they do not permit random access of data.