Multiple exposure is an old photography technique still achievable in-camera in the digital age. The principle is simple : expose the frame multiple time. In film days in meant that you did not advance the roll and take another picture on top of the previous.
This leads to very interesting effect especially when you mix portrait and textures. I like to introduce of few of those effects during couple sessions and wedding as it really makes for unique images. On top of that I pride myself in doing this effect without photoshop. "It's all done in-camera" as a purist would say