The ".ico" extension is the default file extension used for Windows icons. It is a small square bitmap image used to represent a file, folder, program, etc.

The favicon.ico file is used for website icons that appear to the left of the Web address in a Web browser. These files are saved in the root directory of the website on the Web server and are automatically displayed when a user visits the site.

Since ICO files are standard bitmap images, they can be edited with most image editors. If an application will not open an ICO file, changing the file extension to ".gif" may allow the program to open the file.