ICO is an image file format (associated with ".ico" file extension) developed by Microsoft and designed for storing icons. It is widely used in Microsoft products, notably in Windows operating systems. From there it spread to other platforms and Internet. Use this tag if your question is related to this specific format. Do not use this tag for general questions about icons or concepts such as "favicon" if you don't plan to use ICO format for them.
Single ICO file can contain many images of different pixel sizes and color depths. The idea behind this is that software can pick most appropriate format without performing internal scaling, which from historical point of view had impact on performance and from the other point of view may give poor results when it comes to appearance. Scaling icon from high to small resolution, like 16x16 pixels, may require artist intervention or specialized software to keep icon sharp and readable.
