I think it's kind of what sound they aim for. Violin players prefer instruments giving a full, rounded sound, whereas fiddle players go for more treble-y instruments. So that may make some difference. As T says, there may be different construction on a fiddle.And violin players always play with their instrument held under their chin and with (ideally) no weight on the left hand, whereas fiddle players traditionally play with the fiddle held firmly in the left hand, without a shoulder-rest and braced against the shoulder or armpit, or (American style) held in the lap. This isn't always the case though, since most fiddlers will now learn classical violin and switch to playing fiddle music afterwards, so lots now play fiddle with a shoulder-rest.
Grab.