Broccoli is an edible green plant in the cabbage family whose large flowering head and stalk are commonly eaten as a vegetable. Broccoli is, in my view, one of the most vegetabley vegetables there is. It is not sweet at all, instead largely eaten for health reasons. It is fairly dry to eat raw due to its high fiber content and head, and thus it is more often eaten boiled or steamed. It is green. They grow in the ground as part of a plant that just looks like a bigger piece of broccoli. It’s very clearly part of a plant, not a fruit.
It is actually an inflorescence, which is a flowering head. Each little green dot at the end is a flower bud. If picking is delayed too long, each will bloom into a little yellow flower. The stem is literally a stem. Broccoli is definitely a vegetable.
It is also an interesting fact that broccoli is botanically the same species as cabbage, cauliflower, kale, Brussels sprouts, and some other less common vegetables. They are all cultivars of the same species, Brassica oleracea. Broccoli specifically has been bred to have a large flowering head, instead of focussing on the leaves like most other cultivars.