Veggies are a source of various phytochemicals (phyto- meaning plant, and chemicals referring to the fact that they are molecules). Some are naturally occurring in almost all plants, while others are occurring in only some families of plants.
These compounds are sometimes called phytonutrients, phytoalexins, or phytochemicals amongst other things. Each definition differs slightly, but they tend to refer to the same compounds.
Phytonutrients found in a wide range of plants in varying degrees are:
Although there are no panaceas or cure-all tricks in nutrition, vegetables come close. That being said, they alone cannot make the best diet.
Note that there are healthy compounds found primarily in meat that cannot be obtained from vegetables.