ICO is a Microsoft Windows icon format.
ICO files can have multiple icon images of different sizes and color types in the same file, so that programs displaying them can pick the best image for a particular use.
CUR (cursor) files are in a nearly identical format with only a few bytes different.
An ICO file begins with a header, and then an index listing all of the images in the file. For each image, the index contains information about its size, color format, and location in the file.