Essentially there isn’t a difference between a violin and a fiddle in the instrument itself. The name of the instrument changes with how the instrument is played and the style in which it is played.
There can be some subtle differences in the instrument if the instrument is a fiddle rather than a traditional violin. This will be more to do with the set up of the instrument and the strings used. A traditional violin will be set up and used according to a precise guide on strings, bridge set up and it will all need to be just right.
A fiddle is may use steel strings but will still be set up properly. The style of playing on the fiddle will be different to traditional classical music that the violin often plays. The fiddle usually plays music in the style of bluegrass, country or folk but the two are interwoven and either can play any music.
However, at the end of the day the word fiddle is a nickname for a violin and both instruments can play the same music and be identical to the naked eye! Fiddle can also be an affectionate name for a violin and a player may just refer to his/her instrument as a fiddle (again, a nickname!)