MPO files created by digital cameras are captured by the two lenses on the digital camera, then combined and viewed side-by-side as a single 3D image. The image appears 3D when displayed on a supporting 3D device such as a 3D TV. The files can also be converted to an anaglyph image with a stereoscopic image processing program and then viewed with red/green 3D glasses. Standard JPG image viewers can also open MPO files, but you may only see one of the two images, generally the left image.
MPO files may also be created by Nintendo 3DS. The files are 640x480, which is the screen size of the device and they can be viewed using NVIDIA 3D Vision Photo Viewer in Windows.
The MPO format was developed by The Camera & Imaging Products Association (CIPA). The specification actually allows for any number of JPG files to be saved in an MPO file, but the most common use is for stereoscopic images. Digital cameras that support the Multi Picture Object images include the Fujifilm FinePix Real 3D series cameras.
NOTE: Another popular stereoscopic image format is the Stereo JPEG Image format, which uses the .JPS file extension.