LUKS (Linux Unified Key Setup) is a part of dm-crypt which provides encryption for block devices by the Linux Kernel. It is possible to encrypt a whole hard drive or a container which can be stored in a native file system containing unencrypted files.
You will learn how to create such an encrypted container here. Some common scenarios could be to store the container on an cloud service such as (Dropbox, Google Drive, Own Cloud and co.) or to store sensible data own your own devices or at your workplace.
.