Well, I've taken the class, so I guess I'll say what I learned.
When it started, they told us that the main difference between a violin and a fiddle is that a violin comes in a case. That, and if you're a fiddler, teeth are optional.
Jokes aside, the biggest difference I found between classical and fiddling is that you don't learn fiddle music from a score; it's all by ear. In fact, when you get sheet music for fiddling songs, it's called a "Fakebook", because if you need the music then you're faking it. I had not had much previous experience in learning songs by ear, and for me that was the hardest part. It's a wonder, however, how much easier that makes it to memorize the songs, as you have no choice.
Other than that, the differences are mainly in ornamentation. Part of fiddling is that you never play it the way it's written. (Assuming that you're using a fakebook.) You constantly add ornamentation wherever you think it sounds good, with the end result that few fiddlers play the same song the same way. The usual ones include anticipations, trills, mordents, and celtic triplets. There's also a technique called shuffling, where you drone a note from the current chord on a different string.
Also, when fiddling, you tend to stay in the upper portion of the bow, and use specific bowing patterns, which is actually very helpful on the faster songs. One is the Tennessee Shuffle, where you separate the first two notes in a group of four, and slur the last two. Another is the Georgia Shuffle, where you slur the last note with the first two of the next group, and separate the third note. (I might be wrong on those names.)
Another part of fiddling was learning accompianment and percussive techniques, which isn't usual for violinists. On further thought, though, most violinists I know should do this sometime, whether for the rhythm excercise or for the attitude adjustment. Usually you shuffle or drone a note from the chord; however, there's a technique called "chopping" where you sort of hit the higher strings with the part of the bow closest to the frog inbetween shuffles. It makes a sort of scratchy ticking sound, and it took me a lot of work to get it right, but it sounds cool in certain songs.
Then there's "filling", where you improvise notes during the spaces between lines when working with a singer. Usually whoever fills for a given verse takes the solo between this verse and the next one, which is also improvised, or at least an adaptation of the melody.
The group element of fiddling was really different from working in a classical group. Not only do you learn the song together, but you come up with ways to rearrange it as a group. And you often do this with a song you all know already, not knowing each other, just sitting down and starting to play. You actually talk while you're playing, to decide who goes next for the solo, or whatever; it's actually quite social. It's not uncommon for fiddling groups to have conversations right on stage, while performing.
Anyway, I was very pleased with how things went, and found fiddling very enjoyable. I intend to keep up on it.
