"What's the difference between a fiddle and a violin?"
"You don't spill beer on a violin!"
Now, the real answer is slightly more complicated, but that joke just about sums it up: a violin is "fancy," and a fiddle is "folksy." Other than that, they're pretty much the same thing.
So what are the differences between fiddles and violins? The short answer is: nothing. The long answer is a little bit more complicated.
The main thing that makes a fiddle a fiddle and a violin a violin is the type of music that is played on it. Generally, fiddles play folk/traditional genres (e.g. Cajun music, Irish trad, and klezmer), and violins play composition-based genres (e.g. Western classical music, Indian classical music, and jazz). And when it comes down to it, it's all pretty wishy-washy. It's not uncommon to hear a great violinist such as Itzhak Perlman refer to his Stradivarius as his "fiddle," or a bluegrass fiddler talk about the family "violin" that his great-granddad carved by hand in the woodshed.
But are there any differences between a fiddler's instrument and a violinist's instrument?
In some cases, there may be some minor and changeable physical differences between a fiddler's instrument and a violinist's instrument. The words "fiddle" and "violin" are, in their purest sense, referring to the non-changeable parts of the instrument itself. To a layman: the wooden box with the curly-ended board sticking off one end. That part of the instrument is exactly the same, whether it's a fiddle or a violin.
The changeable pieces of the instrument, however, are known as the "set-up," and many fiddlers prefer a different set-up than many violinists. The set-up includes the strings, the tuners, the bridge, and any shoulder rests, chin rests, or pick-ups that a player may choose to use.
With strings attached...
A standard fiddle/violin has four strings. Five-string fiddles do exist, and there are plenty of fiddle-related instruments with other arrangements, but if we're talking about the standard fiddle, we're talking about four strings. Classical players (violinists) generally string their fiddles up with catgut strings, which were traditionally made of sheep intestines but are now often synthetic, wrapped ("wound") with very fine metal. The E string is generally an unwrapped steel string which can be tuned with a "fine tuner" (a small tuner on the tailpiece of the instrument) rather than just the tuning peg. Classical players almost always tune their fiddles in perfect fifths, GDAE.
A GDAE tuning is standard in most fiddle traditions as well, though cross-tunings may be used for multiple purposes (including the ever-popular fiddlesticks) and some genres may have a different standard tuning. Most modern fiddle players, particularly those who play American and Western European fiddle genres, string their fiddles with four steel strings, all of which require fine tuners as well as a tuning peg. Both fiddlers and violinists need to change their strings often, as they break frequently and can lose tone over time.
Take it to the bridge...
Another set-up difference can often be found in the bridge of the instrument. The bridge is a small piece of wood -- usually un-varnished maple -- that holds the strings up off the instrument's body. A fiddle player will often use a bridge that is carved to be flatter than that which a violinist would prefer. The flatter bridge lessens the angles between the strings, which allows the player to play two and even three notes at a time... a desirable thing in many acoustic fiddle genres. This is a matter of preference, of course, as a bridge is fully replaceable and fairly easily changed (it's not glued to anything, it's just held in suspension). Some violinists may prefer a flatter bridge, some fiddlers may prefer a more arched bridge. Generally speaking, though, a fiddler will prefer a flatter bridge than a violinist.
Is it harder to play fiddle music or violin music?The fiddle/violin is an incredibly difficult instrument to play, no matter what genre is being played. Some people will claim that violin is harder, but that is bunk. Violinists often need skills that fiddlers don't, and fiddlers need skills that violinists don't. An advanced player in any genre will be equally, though differently, skilled.
So, that's that! A fiddle and a violin, apart from some easily-changed set-up differences, are the same thing. Now go listen to some fiddle (or violin) music!