The term data storage can refer to anything with information recorded on it. Using this broad definition, a hardback volume of an encyclopedia, an audio cassette of a pop song, and even a piece of paper with random words written on it would all be considered examples of data storage. The most popular definition of the term limits it to only the storage of information on computers and similar devices, and is the definition that is used in this article.

Everything a computer “knows” or is able to “know” is called computer data. This includes e-mails, text files, digital pictures, and databases. Computer data storage can be divided into two main categories: primary data storage and secondary data storage. Each is important, but secondary data storage is usually what people think of when they use the term. Both categories will be explained below.

What a computer “knows” at any given time is technically what information a computer’s central processing unit (CPU) can directly access. This information is called memory, and the components that store it are considered primary data storage. Memory is mainly stored on Random Access Memory (RAM). There are many types of RAM, but they usually come in the form of modules that plug into a specific slot inside the computer. Primary data storage is constantly being erased and rewritten, most often from secondary data storage.

Secondary data storage represents all of the other types of computer data storage not included in primary data storage. Though some experts previous used an additional category called tertiary data storage, technological advances have blurred the differences between the secondary and tertiary levels to the point that only one term is necessary. Internal hard disk drives, CD-ROM disks, and flash memory sticks are all examples of secondary data storage. Because there are so many different types of secondary data storage, this category can be further divided into three different areas: on-site, removable, and off-site data storage.

On-site data storage represents any type of storage device that is designed to remain with the computer or at a single location where the computer is housed. The most common on-site data storage device is a hard disk drive, and it is included in almost every personal computer. Solid state drives and network attached storage are also examples of on-site storage devices.

Removable data storage is any type of data storage that is designed to be easily removed from a computer. Removable data storage has become more common than on-site data storage in modern times. The big disadvantage of removable data storage used to be that data access time was much slower than on-site data storage, but speed improvements have decreased this penalty to within acceptable limits for many common applications. CD-ROMs, DVD-ROMs, USB flash drives, and portable hard disk drives are all examples of removable data storage.

Off-site data storage is one of the most recent types of data storage. In off-site data storage, information is stored away from the computer at a distant location. This data can then be accessed either by a direct call or through the internet. Off-site data storage has the advantage of being available if something happens to the on-site computer system.

Off-site data storage is at a disadvantage to other types of data storage both because of how long it takes to access the data and because it can be less secure than other data storage types. Examples of off-site data storage include electronic vaulting, on-line file hosting services, and on-line photo sharing websites. Off-site data storage is also a primary aspect of cloud computing.