Storage devices store data and programs permanently even when power is turned off. The most common type is a hard drive. Storage is measured in bytes and larger units include kilobytes, megabytes, gigabytes, and terabytes. There are two main types of storage: primary storage (RAM) which is volatile and temporary; and secondary storage which is non-volatile and permanent such as hard drives, optical disks, solid state drives, magnetic tapes, and removable media like USB drives and SD cards.