In general, any storage device that can be attached to a computer through a USB port. The term is particularly used for small portable storage devices typically sealed in plastic. Their physical size is reflected in the various names for this type of device—thumb drive, pen drive, keyring drive, memory stick. They have capacities as high as 4 gigabytes and on most modern personal computers they can be recognized as an additional drive without the need for a special driver. Storage devices of this type are increasingly used for backup, data transfer, storage of photographs or MP3 files, etc.