The division between formats have changed over time, but the medium format typically uses roll film that is 60 mm wide. A 35 mm wide format is miniature, while a 40 mm used to be called miniature, but may be considered medium now.
Upwards, the division is more easily defined, since large format camera use sheet film, not rolls.
Medium format film is used by some professional photographers and a few amateur enthusiasts. Cameras and film for the format are very expensive. The quality of the images obtained, however, is impressive, allowing enlargements to huge dimensions without blurriness. Historically, the format was much more widely used with the lower-quality films of earlier eras. "Classic" medium-format cameras are collected and used by antique camera enthusiasts.
All medium format cameras manufactured today use the 120 film format: