Tea fermented by a Symbiotic Colony of Bacteria and Yeast (the notorious SCOBY). Makeup can get pretty complex but loosely speaking you've got yeast(s), lactobacillus, acetobacter.
Main ingredients are sugar and tea. Lactobacillus eats the tea and makes lactic acid, which gives you your sour taste. Yeast eats the sugar and makes alcohol and CO₂. Acetobacter eats the alcohol and makes acetic acid.
Gallon batch: 7 tea bags, 12 cups water, ~cup of sugar, 2 cups starter