Consisting of high calories, but that is considered only as a broad umbrella. These foods are prepared in a way that they look appealing and are enjoyable so you are chemically programmed to ask for more.
According to Dr. Sunali Sharma, Dietician & Nutritionist, Amandeep Hospital, "Commercial products including but not limited to salted snack foods, gum, candy, sugary desserts, fried fast food, and sweetened carbonated beverages that have little or no nutritional value but are high in calories, salt, and fats may be considered junk foods.
Though not all fast foods are junk foods, but a great number of them are.
For instance, a salad may be fast food, but is definitely not junk food. Some foods like burgers, pizzas, and tacos may alternate between junk and healthy categories depending on the ingredients, calories and process of manufacturing."