They are the same object. A fiddle and a violin come out of the same workshop, take the same strings, and answer to the same four open notes. The question survives anyway, because plenty about the two registers genuinely does differ — just none of it in the wood.
Setup
The most concrete difference. A fiddler often prefers a flatter bridge, which brings neighbouring strings closer to the same plane and makes leaning between two of them easier — useful when the music wants pairs sounding together and the bow arm is busy. A classical setup keeps a rounder curve, which isolates each string and makes a single clean line easier. Neither is a modification of the other; they are two answers to what the player needs most often.
String choice follows the same logic, and so does whether there is a shoulder rest and a chin rest at all. Plenty of traditional players use neither, which changes how freely the left hand can shift — see the setup guide for what is adjustable and what is not.
Bowing
This is where the two really part company. A fiddler's default is a repeating pattern — the shuffle and its relatives — applied without conscious thought, so the groove comes from where the bow happens to change rather than from an interpretive decision. Classical bowing is chosen phrase by phrase to serve a written line. Same arm, same physics, completely different job.
Ornament
Fiddle ornament is rhythmic: cuts, rolls, drones, double stops that thicken a beat. Classical ornament is melodic and largely written down. Vibrato is the clearest tell — continuous in one register, occasional and deliberate in the other, and a fiddler using constant vibrato on a dance tune sounds as out of place as a soloist playing without any.
Repertoire and transmission
The deepest difference and the least often stated: a fiddle tune is learned by ear and has no authoritative version, while a violin part is learned from a page and has one. That single fact explains most of the rest — why the tune pages here publish maps rather than transcriptions, why two settings of the same tune can disagree completely and both be correct, and why "which version is right" is a question the tradition simply does not ask.
What does not differ
The instrument, the tuning, and the geography. Every position frame and every scale map on this site is identical for both registers, because the fingerboard does not care what music you make on it. If you learned one register and want the other, none of that is wasted — what you are changing is the bow arm and the ear, not the hand.