© Denzil Green
Malt vinegar is very popular in England and Canada. In many people's minds, it is the best vinegar to put on fish & chips, and on chips (aka French fries) in general.
It is made from malted barley and grain that is mashed, then fermented, often in beech or birch barrels (or in any event, in casks containing pieces or shavings of those woods.)
Malt vinegar is too assertive for mild vinaigrettes and sauces, but it is good for more robust applications such as marinades, pickling and chutneys.
You can buy regular malt vinegar, which is brown, and distilled malt vinegar, which is clear. Some finer malt vinegars are now being brewed from single malt ales, for aficionados.
In Canada, there's a legal definition for malt vinegar. "Malt Vinegar shall be vinegar made from an infusion of malt undistilled prior to acetous fermentation, may contain other cereals or caramel, shall be dextro-rotatory, and shall contain, in 100 millilitres measured at a temperature of 20°C, not less than (a) 1.8 grams of solids, and (b) 0.2 gram of ash." 
Good for pickling, though, like cider vinegar, malt vinegar can darken light-coloured fruits & veg (unless you use distilled malt vinegar, which is clear).
If an English recipe calls for vinegar without specifying which type, it's generally malt vinegar which is meant.
 Canada. Food and Drug Regulations Consolidation. C.R.C., c. 870. Division 19. 7. B.19.005. [S]. November 2014. Page 755.
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Vinaigre de bière, Vinaigre de Malt (French)