Stack views were introduced in iOS 9 as a way to automatically lay out views in a linear, vertical or horizontal orientation without needing to define constraints. A stack view handles alignment, distribution and spacing of its arranged subviews. Key properties of a stack view include alignment which controls how views are positioned within the stack, distribution which determines how views are sized, and spacing which sets gaps between views. Stack views can be nested to create more complex layouts, though the stack view itself does not determine size or position, only how its subviews are arranged relative to each other.