Data-at-rest encryption
This article discusses data-at-rest encryption software, which on-the-fly encrypts / decrypts data written to / read from a block device, disk partition or directory. Examples for block devices are hard drives, flash drives and DVDs.
Data-at-rest encryption should only be viewed as an adjunct to the existing security mechanisms of the operating system - focused on securing physical access, while relying on other parts of the system to provide things like network security and user-based access control.
For Full-disk encryption (FDE), see dm-crypt/Encrypting an entire system.