Birch tar is produced by the rectification (purification by re-distillation) of crude birch oil, which is itself obtained by the slow destructive distillation of birch bark (a process in which the decomposition of the organic material is achieved by heating it to a high temperature). The birch bark we use is a byproduct of the wood industry in the USA, with the distillation process also taking place in the USA.
Birch tar, which has been used for medicinal purposes since the late Paleolithic era, has a lovely smokiness to it with a woody, almost leathery undertone - a little like an old, full-bodied brandy.