Really a fiddle is a violin, but there's a little more to it than that. Officially the definition of fiddle according to my Harvard's Concise Dictionary of Music and Musicians is any bowed instrument, but I'm pretty sure when I'm asked to fiddle, this is what they mean:
It is a specialized style of music too, just like jazz, and not everyone is trained in those styles.
Common distinctions between violins and fiddles reflect the differences in the instruments used to play classical and folk music. However, it is not uncommon for classically trained violinists to play folk music, and today many fiddle players have some classical training. A lot of traditional (folk) styles are aural traditions, so are taught 'by ear' rather than with written music.
Folk Music: Music in oral tradition, often in relatively simple style, primarily of rural provenance, normally performed by nonprofessionals, used and understood by broad segments of a population and espeically by the lower socioeconomic classes, characteristic of a nation, society, or ethnic group, and claimed by one of these as its own. (pg 236 of Harvard's Concise Dictionary of Music and Musicians.)
Unfortunately for me, I wasn't trained in jazz or fiddling. I can't swing beats or improvise very well... I can play a glissando (a continuous or sliding movement from one pitch to another) which you can hear in fiddle music a lot.
Fiddling is also stereo typically associated with Appalachia, country folk, and bluegrass but it goes deeper. You can hear irish and scottish fiddling in Cape Breton, Nova Scotia, Canada.
It's making some headway in pop culture, I'm sure you've heard of Celtic Woman, well they have they're own minstrel, Máiréad Nesbitt. In fact, they'll be at the Oakdale Theater in Wallingford on March 17th and the Mohegan Sun Arena in Uncasville on March 18th.
I'll leave you with a video of her playing and dancing like a crazy person haha.
Read this and other entries at my blog: http://watercolorhalfnotes.blogspot.com