Primary, Secondary and off-line storage

Primary:
The computer's main memory, which is directly accessible by the CPU and much faster than the secondary storage.

Examples =
Random Access Memory (RAM) - Solid State
(RAM will hold the loaded operating system, plus all running applications and files)
Read Only Memory (ROM) - Solid State

Secondary:
A non-volatile storage that holds data until it's deleted or overwritten. Secondary storage is where programs and data is kept for a long-term basis
(Note: could be referred to as external memory and auxiliary storage.)

Examples=
Hard Disk Drive (HDD) - Magnetic Storage
Solid State Drive (SSD) - Solid State)

Off-line:
Off-line is also non-volatile. Can be easily removed from a computer. Often used to transport data and keep backups for protection (Prevent data loss caused by hard disk head crash)

Examples=
CD, DVD, Blu-ray - Optical storage
USB Flash Drive - Solid state
Removable HDD / SSD