I'm doing a small presentation on the fiddle for a friend's third grade class next week. Being as they are third graders, and thus pretty intimidating, I figured I should do a bit of research on important matters, such as The Question: What is the difference between a fiddle and a violin? My usual smart-aleck answer (which has, I insist some truth in it) is, "Depends on who you ask."
The general consensus is, of course, that it's a fiddle when used for folk music, and a violin when classical music is its provenance. I have, however, heard a good many fiddlers (the name, when applied to people, is less vague) refer without a second thought to, "My violin," (or, "Mah vahlin," in some cases). They play it: they're entitled to call it what they like, says I. There are, of course, extremists on both sides, who hold hard and fast to one term, but in general, I think it's safe enough to say I play the fiddle (sometimes-described-as-a-violin).
Well, it occurred to me to find out where the two disparate words came from. "Fiddle," it turns out, is from the Old English fiðele, a word which may have applied to the rebec, or something like it. Furthermore, fiðele likely descended from the Middle Latin vitula*. I am not certain whether vitula referred to a particular instrument, or whether it was a generic term for stringed instruments at large. But anyway, vitula. Remember that.
"Violin," on the other hand, had its roots in the Italian word viola. Viola, it seems, was once a much more general term than it is today, applying to anything (i.e. what we know as basses, cellos, violas, and violins) with a viola-ish shape. Violino was the diminutive form, applied to the "little viola". The fun part--viola is also quite possibly a descendant of vitula.
So, what is the difference between a fiddle and a violin? Like I said, it depends on who you ask.
*To be thorough, there is also a theory that it is of purely Germanic origin.