Any device capable of storing data is known as a Storage Device. The term usually refers to mass storage devices, such as disks, tapes, etc. Technically, storage devices can be defined as any piece of hardware whose main function is to store data and information. As far as computers go, there are several types of storage devices with many different functions. Storage devices have a different definition and purpose, depending on who is using them.

Computer data storage, often called storage or memory, refers to computer components and recording media that retain digital data. Data storage is a core function and fundamental component of computers.Many different forms of storage, based on various natural phenomena, have been invented. So far, no practical universal storage medium exists, and all forms of storage have some drawbacks. Therefore a computer system usually contains several kinds of storage, each with an individual purpose.

While the hard drive is the most dominant of all storage devices, several other types are common as well. Flash memory drives, such as USB drive and Solid State drives (SSD) are popular ways to store data. Other types of flash memory drives, such as compact flash and SDHC memory cards are popular ways to store images taken by digital cameras.External hard drives, CD and DVD drives are also common. These types of drives are often used for backing up internal hard drives, storing video or photo libraries, or for simply adding extra storage.

There are two storage devices used in computers; a primary storage device such as computer RAM and a secondary storage device such as a computer hard disk drive. Without a significant amount of computer memory, a computer would merely be able to perform fixed operations and immediately output the result. It would have to be reconfigured to change its behaviour.

Primary storage devices, often referred to as memory, are the only one directly accessible to the CPU. The CPU continuously reads instructions stored there and executes them as required. Any data actively operated on is also stored there in uniform manner. This is generally known as computer ram.

Secondary storage devices differ from primary storage devices, in that it is not directly accessible by the CPU. The computer usually uses its input/output channels to access secondary storage devices and transfers the desired data using intermediate area in primary storage. Secondary storage does not lose the data when the device is powered down. In modern computers, hard disk drives and solid state drives are usually used as secondary storage devices.

Tertiary storage or tertiary memory provides a third level of storage. Typically it involves a robotic mechanism which will mount and dismount removable mass storage media into a storage device according to the system's demands; these data are often copied to secondary storage before use.

Off-line storage devices are computer data storage on a medium or a device that is not under the control of a processing unit. The medium is recorded, usually in a secondary or tertiary storage device, and then physically removed or disconnected. It must be inserted or connected by a human operator before a computer can access it again. Optical discs, flash memory drives and removable hard disk drives are examples.