You must have heard about 35mm or 70mm but do you know what these numbers actually mean? Well, here is the answer :
The number preceding the mm refers to the width of the film strip. There are different types of film strip widths – 8 mm, 16 mm, 35 mm, 70 mm and several variations of these.
35 mm is the most common width of film strip that is used in movie theaters. Every time you go to a normal movie theater (the one that has a mostly wide rectangular screen with its width very much larger in proportion to its height), you’re in a 35 mm screen or theater provided of course, the theater uses film projection. The alternative is digital projection where movies are not played from film strips but from hard disk drives.
70 mm film on the other hand, is the one used in large screen IMAX theaters. As you can imagine, the width of the film strip here is 70 mm leading to a larger film strip area. The larger the area, the more detail captured on film and the better the clarity, which is why these 70 mm IMAX theaters have an amazing clarity in comparison to normal 35 mm theaters.
Source : Quora