When land is forested, trees cover it — not two or three trees by a road, but a real stretch of woods. We use it about places: a forested hillside, a forested island, forested country. You would not usually call a city street forested, even if it has trees; people keep this word for land that is more woods than anything else. It is a calm, slightly school-book word. You meet it in geography, nature news, and reports about land. Though actually, speakers also say wooded or forest land for nearly the same idea. Wooded can feel smaller and softer (a wooded garden). Forested sounds like a true forest. There is a verb to forest, meaning to plant trees until land becomes forest, and forested is its past form too — but you will almost always meet it as a describing word: the hills are forested.