"Format" refers to the size of the recording medium in a camera. I say recording medium because the term originated in the film era and has continued to the digital age.

There are no hard limits but medium format is typically anything larger than 35mm film up to 6cmx7cm film. Large format is typically everything from 4"x5" up.

In addition to the size of the film or sensor there are a number of features typical of medium and large format cameras:

Medium format

• Single lens reflex design with pentaprism or waist level 'finders (which look directly onto the focus screen from above and show a flipped image).
• Highly modular design. In additional to interchangeable lenses, medium format cameras usually have interchangeable backs (with film/sensor), and sometimes interchangeable grips, viewfinders.
• Focal plane or leaf shutter.
• Can be manual or fully automatic with autofocus and metering.
• Digital backs available up to about 645 size (53x36mm). Digital backs are just starting to become affordable.
• Were common when 35mm cameras were more primative.
• Used by landscape, fashion and portrait photographers.

Large format

• Lenses mounted on lensboards (instead of traditional locking lens mount).
• Leaf shutter mounted inside lens.
• Bellows design with no reflex mirror.
• Allows movement and tilt of lens.
• Relatively difficult handling, usually requires tripod and careful setup.
• Fully manual.
• Digital backs are available but only scanning backs cover the full format, meaning scenes have to be static or will suffer from shearing. Prices very high.
• Favoured by landscape / architectural photographers.
• Offers highest possible level of detail in images.