Screw it. Get cremated. Faster, and you don’t have to worry about getting dug up later and having alien scientists deduce every nuance about your life and death only to make poorly grounded generalizations about the rest of us.
Always thinking.
I don’t think this is common here in the UK. There is just the wooden coffin which is buried directly in the ground. We also don’t go in for embalming much here either.
My guess is that it’s commercial propaganda on the part of the funeral industry. The same way a real estate agent will insist on being called a “Realtor” and will never say “house,” preferring “home,” even when it would sound unnatural in colloquial speech.
There’s a T.V. commercial that runs in this area advertising the services of a “New Homes Guide” or some such publication in which a couple complains that their acquaintances have bought a great house in a great location when “I never even knew those homes were there.” A real person would say “I never even knew those houses were there.”
But these maps seem to indicate that geography is irrelevant to the use of these terms. So how are the maps interesting?
On the subject of commercial propaganda. In the UK they like to call themselves Funeral Directors , but the rest of the population call them Undertakers.
It makes no nevermind. A rose by any other would smell as sweet.
There is no difference except in spelling and pronunciation. 
I have to say I was intrigued to see that the Batesville Casket Company offer:
I wonder how many times people have dug up their loved ones after 15 years to see if they can make a claim for the coffin degrading whilst under warranty and demanded they be re-buried in a new one. Seems like a highly pointless warranty to me!!
After mulling this overnight it came to mind that a coffin is an old timey appellation for a plain wood burial box.
A casket is a new fangled desigignation for a fancy metal box with decorative handles and soft padding inside for the comfort of the corpse.
The boss is lurking, so I didn’t have time to read all the posts. What I can say is this: when I worked at the funeral home, I was told that a coffin has six sides (plus a lid and a bottom) while a casket has four sides (plus a top and a bottom).
Hmm … that seems to be the opposite of the conclusion of the previous posts.
No, look again. It agrees with posts # 6 & 14. It disagrees with none.
Hugh Rawson’s A Dictionary of Euphemisms and Other Doubletalk claims that ‘casket’ is a euphemism for ‘coffin’.
js: Not wanting to hijack this thread, I’ll say I would love you to start a thread describing your duties. I always wanted to become a mortician, that is until I worked at a medical lab (saw some pretty gory stuff that wouldn’t hold a candle to working with whole dead bodies). But I would like to hear about your experiences.
:: wink wink, nudge nudge::
The Online Etymology Dictionary:
casket
1461, “small box for jewels, etc.,” possibly formed as a dim. of Eng. cask, or from Norm.-Fr. cassette, from M.Fr. casset. Meaning of “coffin” is Amer. Eng., probably euphemistic, first attested 1849.
I’m also under the impression after reading The American Way of Death by Jessica Mitford, that “casket” is a preferred term by the funerary industry because it seems more “meaningful” than “coffin”.