The titile of this post is the photographic equivalent of football's (soccer) off side rule. Very simple in practice but complicated as all get out to the uninitiated.
Focal Length: The focal length of a lens is the distance from its optical centre to the film or sensor when focused on an object at infinity. Of course there is a more thorough explanation and mathematical formula's to support this here and here. From the photographers point of view the main thing you need to know is, the shorter the focal length the wider the angle of view. So 24mm is wider than 35mm is wider than 50mm is wider than 135mm etc.
FOV: Or field of view, is determined by the focal lenth of the lens and size of the film or sensor. So the longer the lens the narrower the field of view. Again for the maths see here.
35mm Equivalence: Im many lens reviews lemses are often quoted with a 35mm equivalent focal length. With the demise of film many question the validity of this comparison. The answer is simple, it gives us a standard, something by which to judge alternatives. For example, by taking the same photo with a 10 mm lens on a full frame digital camera, a 1.6x crop camera such as a Pentax and a 4/3 sensor camera like Olympus will result in very different images. The full grame has a 35mm Eq of 10mm, the Pentax would be 16mm and the Olympus 20mm. For more reading click here.
To my mind there is a vast difference between a photograph and a snapshot.
A photograph is made, the photographer has a vision for the image in mind and sets about creating it. Waiting for the right light or weather conditions and often visiting the scene several times to get it just right.
A snapshot on the other hand is taken as the opportunity arises, often with minimal thought. It is what we do on holiday, when out with family, it aids us in remembering those moments long past.
Having said that, many snapshots are perfect, just like the postcard, whilst some photographs, despite the time and effort, look like a hurried snapshot.
I take mainly snapshots, but occasionally, I get a beautiful photograph.