Dabrah of Fickle Muse has asked a good question, one that puzzles many, including me not all that long ago. It's about the difference between a fiddle and a violin.
Well, the instrument is exactly the same. It's the style of play that's different. Fiddlers play jigs and reels and hornpipes and such: frequently, though not always, at great pace. Violinists play in symphony orchestras etc. That's an incomplete answer, but I think it gives one the general idea. I think we all recognize fiddling when we hear it, but here are two clip of Natalie McMaster, Canada's most famous fiddler. We have tons of great fiddlers but she's the one who everybody knows. In he first, she plays at the Hollywood Bowl. In the second she demonstrates a song at a fiddling school.