I did wonder whether there were any long standing letterboxers whose experience would put me to shame lurking around here, but since the question is now more generalised I feel more qualified to answer!
What is letterboxing? It's most commonly practised (and originally started) in Dartmoor, and shares a lot of things with geocaching. It originally started with just a handful of boxes on the most remote parts of the moor, and a postcard would be left by the person visiting in the box. They would take the postcard they found, write a little message then send it on its way. These days that has mainly been forgotten - instead each box has a rubber stamp, and you collect impressions of these as you find the boxes. You also have a personal stamp which you use to assert you've visited the box.
What are the differences with geocaching? Aside from the stamps, letterboxes also don't tend to contain tradeable items as geocaches do. If you find a letterbox with a rubber stamp, DO NOT take it and swap it for something else! I've seen a few letterboxes where someone's written in the log something like "left keyring, took duck stamp, TFTC!". Not only is this annoying it shows a complete disregard for the hobby and annoys letterboxers no end. Other differences include:
- There's no minimum distance between letterboxes, indeed it's common to find 2 right next to each other!
- While some letterboxes have 10 fig. grid references that can be used sensibly with a GPS, most don't, and usually with letterboxes you have to use a map and compass to triangulate yourself manually. It takes longer and requires more skill, but because of this I find it more satisfying when you find it!
- There's no centralised map / database for letterboxes like there is for geocaches - while there are electronic forms of the catalogue with updates, these are via email, and the normal way is to order a copy of the paper catalogue which you have to do the old fashioned way - see here for details. They come out twice annually at around the time the clocks change (in the UK).
How do I find letterboxes in my area? So to answer the more specific question therefore about where you can find information on geocaches in your area, look at the catalogue! It has a specific section at the back where you can view all boxes in each 1km grid square which is what you can use for finding all the boxes in a specific area.
The other thing to bear in mind is that there are many, many more caches around than are in the catalogue - no-one knows exactly how many are out there, but chances are you will stumble across some by accident that won't be in there. Still mark them down, they still count!
And if you get lots, there's some rather cool looking badges too :-)