The MSDC option in digital cameras stands for Mass Storage Device Class. It is one of the many subclasses of the USB computer peripheral connection protocol.
This class as implemented in digital cameras, allows a connection between a computer and a digital camera by showing the digital camera, when connected to the computer, as a removable disk drive, temporarily attached to the computer. Pictures taken are retrieved using either cut and paste, copy and paste or drag and drop from the digital camera's picture folder onto a real hard drive or other writeable media available in the computer.
In addition, arbitrary data (does not have to be pictures) can be stored in the digital camera's onboard memory to the limit of the memory. The MSDC device class can be implemented by any kind of electronic device which has embedded memory storage in any form such as flash memory, IDE hard drives, etc.