While you can call it a decorative railing, the precise and elegant term is a balustrade. If it’s specifically on a staircase, banister is also a very common and understood word.
That’s called a balustrade or, more commonly, a railing. In some contexts, especially with older or solid-walled structures, it might be called a parapet.
A handrail is a specific part of a railing system. Think of it as a “part vs. whole” relationship. The handrail is the part you grip with your hand, while the railing is the entire structure that serves as a barrier.