Junk food or fast food as they call it, is one of the greatest source of unwanted calories and fats. Junk foods contain empty calories. Empty calories mean calories which are taken into the body without much of nutritious substances like vitamins and essential proteins. These just add up to the calories enormously.
Fats are the tasty components in food. The higher the fat content, tastier the food.
Do not be hoodwinked by the labels on packets like "LOW FAT", "NO TRANS FAT", "HEALTHY FOOD". No junk food is healthy for you. Some rare preparations made out of whole grains are rich in fiber and may actually help to reduce your cholesterol levels and combat obesity.
Carbonated drinks like Coca Cola, Pepsi all are full of calories in each small sip you take. So avoid them completely if your at least planning to get leaner by the year.
There is only one rule regarding junk food:
STILL AVOID IT!
If you really feel to break the rules and go for it once in a while limit it to once in a month and do limit the quantity of food in one sitting.