Rude describes a type of behavior that isn't appropriate and usually isn't very nice, either — like yelling "You stink!" at a children's talent show.
Rude refers to bad behavior or just plain bad manners. For example, children are taught to say "please" and "thank you" or they are considered rude. A rude person needs a little work — rude also means crude or basic, like a rude cabin out in the woods that barely keeps the rain out. It can be a sudden realization, too. If you wake up and see your breath, it's a rude awakening that the heat's not working.