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Computers use primary storage such as random access memory (RAM) and cache to hold data that is being processed. This type of memory is volatile – it loses its contents when the computer is switched off. However, general purpose computers, such as personal computers and tablets, need to be able to store programs and data for later use or permanently.

Secondary storage is non-volatile, long-term storage. It is used to keep programs and data indefinitely. Without secondary storage, all programs and data would be lost the moment the computer was switched off.

There are many forms of secondary storage. Each type of secondary storage device has its own characteristics. Because all devices are different, some are more suited to certain applications than others.

For example, a hard disk drive has a high capacity and reasonable access speed, making it suitable for everyday storage of programs and data. A USB memory stick is smaller in capacity, but extremely fast and portable, making it suitable for transferring files between computers.

Not all computers require secondary storage. Embedded computers, such as those found in a smartwatch or central heating system, do not need to store data when the power is turned off. The instructions needed to run them are stored in read only memory (ROM) and any user data is held in random access memory (RAM).