Junk food is any variety of unhealthy processed food. Fast food is the food you get from quick service restaurants that serve things like burgers, fried chicken, etc.

Junk food = French fries 🍟 or soda 🥤 Fast food = burger 🍔 etc Excellent question.

Junk food and fast food are practically synonymous. Fast food is a junk food (= with high calorific levels yet low nutritional value). Most junk food could easily be served fast as they tend to be ready to 'heat and eat'. Distinctions are mostly subjective. Imagine a Venn diagram of junk food versus fast food. Most items would be in the overlap.