Malt vinegar is essentially made from unhopped beer: that is, from cereal grains and sprouted barley. It has overtones of barley malt. This was the standard form of vinegar in beer-drinking Britain, where it was originally called alegar.
From the book On Food and Cooking (2nd edition) by Harold McGee. © 2004 Harold McGee. By permission of Scribner, a division of Simon & Schuster, Inc.