What is ICO?
Windows icons, and a container rather than an image - one file holds the same icon at several sizes, so Windows can pick the right one for a title bar or a desktop. It tops out at 256x256 and supports transparency, which is what stops an icon looking like a sticker.
What is ICNS?
Apple's icon container, the thing inside every .app bundle that makes it look like something. Like ICO it holds a set of sizes, but it goes considerably larger - Retina displays ask for 1024px icons, which Windows never did.