If you’re playing country or bluegrass, it’s a fiddle. Otherwise, it’s a violin.
Are they really the same instrument or is there some difference in the way they’re manufactured?
Nope, they’re the same thing. I knew a lot of violinists who would play classical and fiddle music on the same instrument.
Yep, in most cases, fiddle = violin. However, there’s exceptions, such as the Scandinavian Hardanger Fiddle which makes use of sympathetic strings (which are not played directly, but which resonante with the main strings), and the Suka, a Polish fiddle not really like anything more familiar in Europe, played upright and using the fingernails to stop the strings.
Oh, and I should add that ‘fiddle’ applies to plenty of European folk music as well as American ones. And also, that folk music often involves a very different technique to classical violin.