Junk food is
food which, while technically digestible (and often
tasty), contains disproportionately little
nutrition for the amount of
fat and/or
sugar involved.
Junk food includes candy, chips (in both the international and US meanings) and crisps, baked goods, and some (if not all) fast food. Soda/pop is usually considered junk food rather than a junk beverage.
Organic, all-natural stuff can still be junk food. Newman's Own chocolate is tasty, it's sold in health food stores, and yet it's junk food. Ditto Red Hot Blues and other salty snacks.
Note that junk food is not necessarily the opposite of health food; ordinary vegetables, fruits, cereals, grains, eggs, fish, meat, etc. are neither health food nor junk food.