Double exposures occur when one exposed frame of a photographic negative overlaps another exposed frame, either completely or partially. These overlapping frames form an area on the negative that has been exposed twice, thus the term "double exposure". Above are some double exposures that I took in South Africa this past holiday season, shot on both 120 b&w film and 35mm color film with a Diana F+ toy camera and a Diana Mini toy camera, respectively.
My double exposures were made by shooting two frames in-camera onto the same area of a negative, but these results from a traditional method can also be accomplished now in Adobe Photoshop or in-camera on Digital SLR camera bodies. Here is a video that details the Adobe Photoshop method, enjoy!