Information is stored when it is at rest. Usually on a device such as a hard drive, memory, CD/DVD or thumbdrive.
What do CD-ROMs, DVDs, USB drives, and hard-drives all have in common? They all retain their data, even when you shut them off. These devices are known as non-volatile memory. Volatile memory loses its content when it loses power, such as the RAM in your computer. That’s why we don’t store all our files on RAM: we’d lose our files every time we shut down our computer!
Storage devices can be secured in a number of ways: preventing physical access, implementing logical encryption, and following proper procedures to wipe data on the media if the media is disposed of or re-purposed.