In short, junk food is any kind of food not intended to be part of a balanced nutrition. It doesn't have to be "bad" for you, it just has to be something created mostly for satisfaction than nutrition.
Wikipedia says the following...
Junk food is a common term used for any food item that is perceived to be unhealthy and has low or poor nutritional value.
I could accept the notion that most food perceived to be unhealthy or of low nutritional value to be junk food. However, consider celery. It's mostly water, and is actually low in nutritional value. But we wouldn't consider celery to be junk food.
On the opposite end, consider unseasoned, unsalted, rice cakes. There's nothing unhealthy about these things, in fact, they're pretty good eating as far as diet and health are concerned. But I feel they straddle the line on junk food. Rice cakes were made for snacking, or at best, a platform to smear jelly or cream cheese. They're more about satisfaction than nutrition.
You could also say the same thing about potatoes. By themselves, they don't offer much in terms of essential vitamins and minerals. And, no one eats potatoes plain, with nothing on them. We always pile on butter and sour cream. Other people sprinkle salt and pepper. We dab french fries into ketchup, and mix cheesy sauce with Au Gratin style. In that sense, they're not much different than rice cakes.
So, would we consider potatoes to be junk food? No. Potatoes by themselves were not created purely for satisfaction. On the other hand, processed potatoes, like french fries, hash browns, potato chips, are junk food. I'd have to exclude baked potatoes from junk food, because you gotta cook a potato somehow.
Pizzas are generally considered to be junk food. But what part of a pizza makes it junky? The bread? The cheese? The tomato sauce? The pepperoni? If you answer "The pepperoni", then what if we exchange it for mushrooms? Would that make it healthy? I'm not sure why pizza is more junkier than a plate of Chicken Parmesan over Linguini.
As for Junk Food Blog, what's the criteria for getting a product blogged here?
It's really up to me to decide, mostly based on what I feel is good. But here's a general set of criteria I look for...
1. It is "ready to eat" (straight out of the package).
2. It is not marketed as a health food.
3. It is consumed as a snack, not part of a meal.
4. It is manufactured.
I may stray off of these rules, if it's something I can't resist telling you about.