A fiddle is the same thing as a violin. The instrument, which has strings and is played with a bow, is more likely to be called a fiddle when it's used to play traditional or folk tunes, rather than classical music.
Fiddle music is typically written for dancing, and it comes from backgrounds as diverse as Scotland, Eastern Europe, and the Cajun and Zydeco traditions from Louisiana. To play the fiddle is to fiddle, and when you mess or play around with something, you also fiddle: "Do you have to fiddle with your pen while I'm talking?" Someone who's "fit as a fiddle" is in perfect health.