An ICO file contains an icon, which is typically used for a Windows program, file, or folder. It stores one or more images in various sizes so they can be appropriately scaled depending on their use. ICO files are similar to .CUR files, which are also used in Windows, and .ICNS files, which are used in macOS.
ICO file open in File Viewer Plus 2
All icons for executable files in Windows are saved in the ICO format. Windows ICO files typically store images ranging from 16x16 to 256x256. Older ICO files only include images up to 48x48 which was previously the recommended dimensions by Windows. However, most newer icons include images up to 256x256 to accommodate screens with higher DPI.
Each image in an ICO file with less than 32 bits of color depth stores two bitmaps: 1) the AND bitmap - the image mask (which determines what part of the icon is transparent) and 2) the XOR bitmap - contains the icon which is mapped onto the image mask. Images with 32 bits of color are 24-bit images with 8-bit channels for alpha compositing purposes.
The favicon.ico file is used to store a small website logo that appears in the address bar of a web browser. If a website has a favicon.ico image, it will appear to the left of the web address when any page of the website is loaded. The favicon.ico image file must be 16x16 pixels and stored in the root directory of the website in order for the web browser to recognize it.