A product of Spring Valley, junk food is very low in nutritional value and should be eaten in moderation. The box encourages people to "see the moon map offer on the back!" (even though there is no such offer).
Junk food is often found in lounge areas or kitchens throughout the wastes. It's very lacking in replenishing hit points and will add a small amount of radiation. In fact, in Fallout: New Vegas's hardcore mode, it doesn't even lower Starvation like all other foods.