Basically an arse is on the opposite side of your anatomy to your meat and two veg whereas an ass can be found pulling a cart and is often refered to as a donkey.
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Ass was dumbed down to mean your gluts. It thus became a swear. So people made up the word "arse" because it seemed more polite, but since it came from arse, it means ass, still. Both mean donkey. Neither really refer to your rear.
"D-o- -u - -u-n-d-e-r- -s-t-a-n-d- t-h-i-s"
BTW, it's "understand."
Someone quick! Change their avatar to a donkey so we can keep up the hissing match!