35MM Film
35mm film is the basic film format most commonly used for both still photography and motion pictures, and remains relatively unchanged since its introduction in 1889 by Thomas Edison. The format was initially called Edison size. The photographic film is cut into strips 35 millimeters wide, with six perforations per inch (25.4mm) along both edges. The origin for the 35mm size is an Eastman Kodak 70mm roll film for photography, being cut in two. William Kennedy Laurie Dickson, working for Edison, then cut four round perforations per frame along both edges. The flattened perforations were introduced by Bell & Howell around 1900, which remain to this day for camera original film.
In the conventional motion picture format, frames are four perforations tall, with an aspect ratio of about 4:3. Still cameras in 35mm and the Vistavision motion picture format use a horizontal frame which is eight perforations wide, resulting in a wider aspect ratio of 3:2 and greater detail, as more film area is used per frame.
The aspect ratio of an image is its width divided by its height (usually expressed as "x:y").
SOURCES:
http://en.wikipedia.org/wiki/35mm_film
http://www.widescreen.org/aspect_ratios.shtml