What is DDS?

DirectDraw Surface, which is how game textures are stored for the graphics card. Block-compressed so the GPU can read it without unpacking, and usually carrying a whole chain of mipmaps. Open a game's files and this is most of what you'll find.

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.