Acids are for instance substances that make liquids taste sour:

   You taste lactic acid  in curdeled milk, sauerkraut or bread made from leaven.
   You taste acidic acid after using vinegar.
   Malic acid and citric acid are not only present in apples and lemons but also in sour cherries.

Acids  are liquids with a pH range between 0 and less than 7.

All acids are proton (H+) donors:   Acid-base reactions.