A collision shape is only a shape. It doesn't do anything other than define an enclosed region. An Area2D is one of the CollisionObject2D types - things that can detect collisions.
When you add an Area2D, it doesn't have a shape. Is it round, square, etc.? The CollisionShape2D lets you define that. Its name comes from the fact that it's a shape which is added to collision objects.
I recommend you read this to get an understanding of the various collision and physics nodes: