Even though it contains some of the most memorable images in human history, how many of us really know how film actually works? Let's break it down to the components. First, those little holes along the side of film strips are called perforations (perfs for short). The sprockets inside cameras and projectors latch onto those to move the film along at a steady pace. Sometimes these holes are on both sides of the film, but in other cases one set of perfs is replaced by a magnetic or optical strip for sound recording.
Between these two rails are the frames themselves, which are measured in a whole slew of confusing ways. The width of the film is what most people are familiar with (8mm, 16mm, 35mm) but it doesn't tell the whole story: some formats take up more space on the strip, the number of perforations per frame are also important and some are even oriented horizontally! They all have different uses, which partially explains why filmmakers are so obsessive about film stock.
Regardless of which stock is in the camera, film exposes more or less the same way. Different layers of the film are coated with chemicals sensitive to either red, green or blue light. With all three stacked on top of each other, the exposed film is able to reproduce the full spectrum of visible light.
Depending on what point in the history of cinema you're talking about, film was made of either acetate, polyester or nitrocellulose — which was abandoned around 1950 because it was really, really flammable (if you've seen Inglourious Basterds, you already know this).
While some directors still opt to shoot on film, we're reaching a point where digital is able to perform as well as or better than analogue, and often at a fraction of the cost. With Academy Awards for both best cinematography and best direction, The Revenant — which was shot digitally with Arri Alexa 65s — might be the harbinger of film's dying days.