A storage device is a hardware device designed to store information. There are two types of storage devices used in computers; a 'primary storage' device and a 'secondary storage' device.
Primary Storage Devices
Also known as internal memory and main memory, primary storage is a storage location that holds memory for short periods of times while the computer is on. For example, computer RAM and cache are both examples of a primary storage device. This type of storage is the fastest type of memory in your computer and is used to store data while it's being used. For example, when you open a program data is moved from the secondary storage into the primary storage.
Secondary Storage Devices
Also known as external memory and auxiliary storage, secondary storage is a storage medium that holds information until it is deleted or overwritten regardless if the computer has power. For example, a floppy disk drive and hard disk drive are both good examples of secondary storage devices. As can be seen by the below picture there are three different types of storage on a computer, although primary storage is accessed much faster than secondary storage because of the price and size limitations secondary storage is used with today's computers to store all your programs and your personal data.
Finally, although off-line storage could be considered secondary storage, we've separated these into their own category because these types of media can be easily removed from the computer and stored elsewhere.