If you are wandering around a video store, the term 'letterboxing' refers to the practice of transferring a movie to a format to be viewed on a television screen, while keeping its widescreen dimensions intact. Since the television screen's dimensions are different to those of the movie screen, movie images shown on television may be distorted or closely cropped to make them fit. A 'letterboxed' version of a movie keeps the original shape of the film image by using the full width of the television screen and leaving the top and bottom sections of the television screen blank.
If you are wandering around the moors of south-west England, the term 'letterboxing' refers to the activity of those people you see dotted about with notebooks and runny noses. They are deciphering sets of cryptic clues in search of plastic cases containing rubber stamps, inkpads, and notebooks. Once found, these 'letterboxers' open the cases, use the rubber stamp to place an image into their notebook, and use a rubber-stamp of their own to place an image in the newly-discovered notebook. There are hundreds of these plastic boxes scattered around Dartmoor National Park just waiting to be uncovered and enjoyed.
If you mention any similarity to trainspotting, they will probably sniff loudly, stamp their insignia on your forehead, tell you to go away and hike away in a huff.
If you mention trainspotting to your fellow shoppers in the video store, they will most likely tell you: 'That's the one with that guy from Star Wars in it.'