Traditional Balsamic vinegar is created from the juice of white grapes (typically, trebbiano grapes) boiled down to approximately 30% of the original volume to create a concentrate, or must.  This goes through a long aging and fermentation process which creates the distinct flavor of balsamic.  The flavor intensifies and deepens over time, with the vinegar aging in a variety of progressively smaller fine wooden casks.  During this period, a portion evaporates; it is said that this is the "Angels' share".  The result is deliciously sweet, viscous and very concentrated.