It's the style of music you play. Some styles call the instrument a violin, e.g. classical, Jazz, Indian, Persian, etc. Some styles call the instrument a fiddle, e.g. bluegrass, Irish, nordic, etc. You'd never talk about Irish violining nor would you talk about Bach's Concerto for two fiddles.
Quick one liner:
"What's the difference between a fiddle and a violin?"
"A fiddle is fun to listen to."