Which acid gives vinegar its taste?

Acetic acid, produced by bacteria that oxidise ethanol - so vinegar is what happens to wine or beer left open to the air, and the French name is literally sour wine. It was probably discovered the moment anyone left an alcoholic drink too long, which puts it among the oldest manufactured products.

Its uses ran far beyond food. Vinegar was the standard antiseptic for wounds for millennia, it dissolves limescale and mineral deposits, and Roman soldiers drank posca, a mix of vinegar and water, which was cheap, safe because the acidity killed pathogens, and effective as a thirst quencher - the sponge offered to Jesus on the cross was most likely a soldier's ration rather than a mockery.

The mother of vinegar - the slimy disc that forms in a bottle - is a cellulose mat produced by the bacteria, and it is harmless. Balsamic vinegar of the traditional kind is a different proposition entirely: cooked grape must aged in a sequence of barrels of different woods for at least twelve years, and sometimes far longer, with a small amount decanted each year and topped up from the next barrel, so a bottle contains material of many ages at once.