Flowers are any plants which grow to a certain height, and then stop growing. While most flowers grow to between .5m and 10m, there are some flowers which can grow to be up to 30m high.
Trees are any plants which can grow without limit. As long as they have adequate nutrients, daylight, and water, a tree can grow exponentially.
Terranean plants are those which live primarily above-ground.
Subterranean plants are those which live primarily below-ground.