There's an old joke, stop me if you've heard it...
"What's The Difference Between A Violin and a Fiddle?"
- You Don't Spill Beer on a Violin!
The violin, viola, and violoncello (AKA cello) are all members of the violin family of instruments, and when speaking of all three, it is usual to simply say “violins”. Fiddles are really the same instrument. Fiddle is a folksy way to say Violin, but there certainly can be distinguishing factors.
Players who specialize in certain folk styles prefer instruments set-up to make it easy to play those genres. This can include utilizing different strings, tuners, adjusting the shape of the bridge, adding or subtracting shoulder and chin rests, and installing pickups for amplified play.
You will find many bowed string players referring to their very expensive violins as fiddles, and vice versa. In the lingo of string players, the terms are often used interchangeably.