Between: preposition - reciprocal action or relation involving two or more as individuals, reciprocally, on the part of; motion or communication from each of two or more bodies, places, etc., to the other or others, to and from; confinement or restriction to two or more parties; in the interval separating two points of time, events, etc.; intermediate to, partaking of the nature of two qualities; in or through the space, line, or route separating two points, objects, etc. or bounded by; in the space bounded by two points etc., dividing, separating, connecting, uniting. Adverb - in or into an intermediate position or course, intermediately in amount, order, occupying intermediate space, intervening; in the interval or at intervals. to go to and fro as a mediator;
Among: preposition - in the assemblage of, surrounded by and grouped with; surrounded by the separate members, components, or particles of, amid; in company with; in the number or class of, in comparison with; in the general practice or views of; divided between, shared by.
Both words have possibly more meanings than most people appreciate. However, the confusion in usage arises only in certain circumstances: i.e. which should be used when referring to two objects and which belongs with three or more things. But this is to over simply the matter. The distinction doesn’t rely on whether you’re referring to two things or to three or more; it’s whether you’re referring to one thing and another, or to a collective or undefined number.
‘The gossip spread among the journalists faster than wildfire.’ But, you could also write, ‘The gossip spread between the journalists like wildfire.’
‘A five bar gate rested between the gateposts.’ Means something entirely different from, ‘A farm gate rested among the fence posts.’ In the former, we have a gate in situ with the posts at either side to support it. But in the latter, we have a gate as a separate item awaiting use along with some fence posts.
There is also a question of the mood conveyed by the two words: ‘The lovers walked among the bushes.’ indicates a meandering stroll. But, ‘The lovers walked between the bushes.’ is suggestive of a more formal promenade with bushes at either side of them.
One other aspect that needs a few words is the use of ‘between’ when used to measure something in relation to the objects the measured item lies between. So, the ‘distance between the posts’ does not include the width of the posts themselves but merely the space intervening. When something falls between two or more other things, those other things are not part of that something, but the defining boundaries of the space into which the something falls.
Zemanta could find no suitable illustration for this one, so here's one of my own, taken on the Wolds, near Bridlington.