Really any data you may want to keep.
Unfortunately, the primary memory that the computer has the system RAM and CPU cache is all volatile.
Volatile memory loses all data that it holds when the computer turns off.
To meet this need, secondary memory is required.
This memory is generally left permanently connected to computers, usually storage devices.
Technically, the same class of storage devices may also be used as tertiary or quaternary memory.
That is storage devices that arent connected but that the computer can connect.
And storage devices that arent related and need manual human intervention for the computer to be able to access.
Storage devices may be intended primarily to be static.
However, they may also be removable.
In HDDs, magnetic fields in a disk platter are aligned or misaligned with a write head.
The magnetic fields can then be reread with a read head.
SSDs, or Solid-State Drives, are the up-and-coming king of storage media.
They utilize high-speed Flash memory that can operate far faster than an HDD can.
Unfortunately, being cutting-edge technology, SSDs have a price premium.
USB thumb drives and external USB SSDs also use flash memory.
Though the bandwidth of the USB connection typically limits that.
Optical storage media such as CDs, DVDs, and Blu-rays are somewhat similar to HDDs.
Optical media suffer from the same speed limitations as HDDs due to moving parts use.
A smaller laser wavelength means that more minor grooves can be detected.
They can be packed closer together, increasing storage capacity.
Historical Storage Devices
One of the earliest technical forms of storage would be the punch card.
However, a computer would generally not have been expected to read the output result from another computer.
This memory was used on the Apollo spacecraft that landed on the Moon.
It worked on the same principles as a hard drive but had much smaller capacities and slower speeds.
It was sold in two roles, an SSD and a cache for other storage devices.
The SSD products were generally considered high-end SSDs.
Magnetic storage tape has historically been used as archival media.
While tape is likely still in archival use, most archival data is now stored on HDDs.
Conclusion
Storage devices are forms of secondary computer memory that can permanently store data.
Over time the storage density of storage devices has come down dramatically.
