Primary storage is typically memory that is volatile (meaning it requires power to keep data) and holds data that is in active use (the operating system, any applications that might be open, and user files that may be edited or read), the most common example of primary storage is RAM. Primary storage is much faster to access then any other type of storage.
Secondary storage is memory that is non volatile (which means the computer can be switched off and the storage will retain the data) and is used to store things like the operating system and any user data/applications when the computer is off, secondary storage typically has a much larger capacity then primary memory. Secondary storage examples include a hard drive or SSD.
Off-line storage is typically comprised of secondary storage devices that are stored in a separate location from the main computer, this is to ensure that in the event of an accident (e.g. A fire that damages the computer) there is a backup of the most important data. An example of off-line storage could be a service like dropbox (Note that off-line refers to not being on site and not a connection to the internet).
Summary: Primary storage is fast and small, Secondary storage is large and slow, Off-line storage is secondary storage that can't (always) be written to (as the drive is in a separate location) and is more secure.
