Medium format film

Each format makes an image with one side equal to 6 cm (2 - 1/4"). 6cm = 60mm

Also called "120" and "220" and 645 and 6 x 4.5 cm and 6 x 6 cm and 2-1/4" and 2-1/4 x 3 - 1/4" and 6 x 7 cm and 6 x 9 cm and more.
All these cameras use the same rolls of 120 and 220 film. They just space out the images differently which leads to all the different sizes.

120 film was introduced about 1902 and is still very popular today. 120 film is a black paper roll with a strip of film taped inside which is drawn through your camera as you shoot. The paper has markings on the back so you could advance the film by looking through a red window in the old days before rapid wind levers.

These markings were for 645 (16 shots) , 6 x 6 (12 shots) and 6 x 9 (8 shots) formats.

The black paper lets you load the film in daylight. To load a new roll of film you move the empty spool left from the previous roll to the other position and put the new roll in its place. You thread the roll onto the empty spool, wind the camera till the arrow on the backing paper aligns with an index on the camera. Close the back and wind till you get to frame one. Some cameras even have automatic indexing so you don't need to fiddle with aligning arrows.