A fiddle IS a violin, yes.
Two different names for the same thing. In general, classical musicians call it a violin, other styles refer to it as a fiddle.
Okay, that makes sense. Seems like "fiddle" is the name used in the Appalachian tradition. I wonder if that means the origin of the term "fiddle" would trace to the use of violins by Scotch-Irish immigrants to America.
Trying to find a history of the mandolin now--here's one:
A Brief History of the Mandolin
By the Seventh Century AD a folk lute called the oud was in use. The oud remains in use today, virtually unchanged, in the music of the Near East, particularly in Armenia and Egypt. 'Oud' is the Arabic name for wood, and the oud is a wooden lute. The oud found its way into Spain during the Moorish conquest of Spain (711- 1492), to Venice through coastal trade, and to Europe through returning Crusaders (around 1099).
In a gallery in Washington, a painting by Agnelo Gaddi (1369- 1396) depicts an angel playing a miniature lute called the mandora. The miniature lute was probably contrived to fill out the scale of 16th century lute ensembles. The Assyrians called this new instrument a Pandura, which described its shape. The Arabs called it Dambura, the Latins Mandora, the Italians, Mandola. The smaller version of the traditional mandola was called mandolina by the Italians.
Interesting--I think guitars also originate from Spain during the period mentioned above, which might fit with guitars and mandolins being related.
I heard Itzhak Perlman once refer to his VEY expensive violin as his fiddle. Then he proceeded to play something that sounded very much like Klezmer music on it!