“Junk food” generally refers to foods that contribute lots of calories but little nutritional value.
One problem with junk foods is that they’re low in satiation value — that is, people don’t tend to feel as full when they eat them — which can lead to overeating. Another problem is that junk food tends to replace other, more nutritious foods. When people drink lots of soda, for example, they are usually not getting plenty of low-fat dairy or other healthful beverages like green tea or orange juice. When they’re snacking on chips and cookies, they’re usually not loading up on fruits and vegetables.