Medium format refers to the film utilized in still photography that creates pictures bigger than 135 film, but smaller than the large format of 4” x 5.” However, when you are speaking of digital photography, medium format typically relates to the cameras that use the sensors bigger than the typical 35mm frame.  In addition, some medium format film cameras can be converted to digital cameras by using digital backs.  On the other hand, medium format in moving film used to be widely popular until the 1950s.  Using medium format in film is still popular, but is certainly more a professional niche than its previous industry standard.