'What's the difference between a fiddle and a violin?' "
When you are buying one, it's a fiddle. When you are selling one, it's a violin.
It's the nut who's holding the bow.
$125 per hour and a tuxedo.
You can't play a violin barefoot.
It's OK if you spill beer on your fiddle.
A violin has strings, and a fiddle has strangs.
About $5,000.
You'll never find a violinist with a mullet.
A violin sings, but a fiddle dances.
It's a matter of style. If you have style, it's a fiddle.
Sooo….When it comes down to it, the real difference between a violin and a fiddle today is a matter of style. If you are playing by ear, and free to improvise, it's fiddling. If you are playing what is on the page with accuracy and precision, it's violin.
Traditional folk styles would fit under the realm of fiddle. Fiddling is learned primarily by ear. It requires internalizing of the melody, and allows for some level of freedom in choosing how you want to present the tune. You can bow it with the bowings you choose, and even the notes you choose, provided you honor the tradition you are following. If you are only playing notes written on the page, you probably don't really know the song yet. But once you know the song, then you have freedom to improvise with it.
I would also include playing violin with country, bluegrass, jazz, or rock music in the fiddle category. They are styles that are fundamentally improvisatory in nature, and allow for clear freedom of expression in your playing.
If you are playing violin in an orchestra, there is no room for improvisation. You must play with the same bowings as the rest of your section, or you will stick out like a sore thumb. You must play the same notes as the rest of your section, or it will sound like a giant mess.
But, even within the classical violin tradition, there is room for fiddling around. If you are the soloist, you have the freedom to bow it how you want, and you even get a cadenza now and again.