What is a healthy diet?

My daughter has decided she wants to eat in a more healthy way. She knows fruits and vegies are good, although she already eats them. In any case she has asked me to buy more berries. I don’t know how good the out-of-season berries are, healthwise. She is doing this because she has been eating a lot of cookies for snacks, and she wants to cut that down. Also, she wants to reduce the amount of ice cream she is eating

Finally, she is thinking of doing more exercise. She did some tonight, but we’ll see how long that lasts. It’s a good thing, anyway.

But what is healthy, as far as diet goes? Is it all about fat and the right amino acids and anti-oxidants and stuff that keeps cancer away? Is it about being a vegan or a vegetarian? Is it about weight or something else? Are there other things to be considered when figuring out what “healthy” is?