Brittany is the top-left, tongue-shaped, bit of France.
When the Romans were there, they called it Armorica and it was inhabited by Gauls. By the 4th century tribesmen from Britain had begun to cross the Channel to settle there, a flow which accelerated during the 5th century as Roman troops and authority were withdrawn from Britain, and Anglo-Saxons entered the country from the east.
To distinguish it from Britain proper, the region acquired the name of Brittany or Lesser Britain.
As time passed the region divided into three kingdoms -- Domnonia, Cornouaille and Bro Waroch (the first two taking their names from the Roman for "Devon" and the French for "Cornwall") -- which were later amalgamated once more, to form the Duchy of Brittany.
The Duchy maintained its independence from the French Crown until 1488, when the duke's army were defeated and the duke forced to concede to the King of France the right to determine the marriage of his daughter and heir. Duchess Anne, as she became, was married to Louis XII of France and although the duchy passed to her daughter Claude, Claude's husband, Louis' cousin, Francois, incorporated the duchy into the kingdom when he became king, as Francois I.