Thanks to http://www.markus-gattol.name/ws/dm-crypt_luks.html#motivation_behind_encryption
Block-layer encryption, also known as “whole disk encryption”, “on-disk encryption” or “full-disk encryption” is a kind of disk encryption software or hardware which encrypts every bit of data that goes onto a disk, disk partition or disk volume of some sort (LUN, RAID Volume, ordinary disk, etc.). The term “full-disk/on-disk encryption” is often used to signify that everything on a disk is encrypted, including the programs that can encrypt bootable operating system partitions. Block-layer encryption is in contrast to filesystem-level encryption, which is a form of disk encryption where individual files or directories are encrypted by the filesystem itself. The enterprise-class block-layer encryption for Linux goes by the name dm-crypt. There is also an extensions to it called LUKS (Linux Unified Key Setup) which enables us to do fancy things like key management for example. dm-crypt and LUKS, both are free-software working together in order to provide data encryption on storage media thus allowing that what is a secret stays a secret. dm-crypt is a device-mapper and part of the Linux operating system kernel. LUKS is a hard disk encryption specification, represented by cryptsetup, its actual implementation. This page discusses the motivation behind encryption and provides a guide in order to setup and manage encrypted storage media.