video by Ted Forbes
video by Karl Taylor
Medium Format photography generally applies to film and digital cameras that record photos on sensors (of film) larger than 24 by 36 but also smaller than 4 by 5 inches.
A 'full-frame' camera is one with a sensor of 24 by 36mm or with 35 mm film. A 4 by 5 or larger generally refers to large format photography.
Photographers use medium format for a variety of reasons but mainly due to the uncompromising image quality and the massive dynamic range.