There's so many opinions on that it's extremely hard to get a clear picture. The "optimal" diet currently in vogue seems to be a sort of squirrel diet consisting of soy milk, legumes, nuts, whole grainS, chicken and vegetables while cow milk, red meat, potatoes, sugar and salt is the spawn of the devil.

In a more moderate sense (i.e. My own biased opinion) "eating bad" is what everybody knows deep down: excess of candy, ice cream, soft drinks, fast food or the tendency to over-eat i.e. All-you-can-eat places and too big dinner plates at home :)

Nothing precise really, but I would say a lot of sugar (candy and soda) would be bad. Vegetables would be good. Avoiding fast foods and sugar will go a long way I'd say.

But other than that I'm really not sure, there are a lot of different (often conflicting) ideas of what is "good" and what is "bad".

Fat is bad vs Fat is good. Calories matter, the rest don't vs Calories are different. etc.

Eating a varied diet of real whole foods and staying away from processed, packaged food is the general consensus for a "good" diet.

Simple sugars above all else; when in doubt, eat more vegetables.

For more, see Why We Get Fat: And What To Do About It by Taubes.