What's the difference between a violin and a fiddle?

Nothing! They are the same instrument. What makes a fiddle a fiddle is the person playing it and the way it is set up. Fiddlers, who mostly play bluegrass and folk music, tend to prefer flatter-arched bridges so that they can bow two strings at a time. Violinists, who mostly play classical or pop music, prefer the traditional, more highly- arched bridge. Fiddlers mostly prefer to play with loud, steel strings, while violinist usually prefer more mellow synthetic or gut strings.