ICO is a Microsoft Windows icon format. Among other uses, it is the only format supported by the Internet Explorer browser for Favicons, though other browsers support a wider variety of graphic formats for this use. Although the official MIME type, in Microsoft's own vendor space (image/vnd.microsoft.icon) has been registered since 2003, it has yet to be supported in MSIE; webmasters still need to serve their favicons as image/x-icon for it to work correctly in that browser.
ICO files can have multiple icon images of different sizes in the same file, so that programs displaying them can pick the size that is appropriate for a particular use. Image data within the file is either in BMP or PNG format, with some header bytes defined by the ICO standard.
CUR (cursor) files are in a nearly identical format with only a few bytes different.