Files with the extension ICO contain icons, ie Small graphics that represent the representation of files or directories in a graphical user interface (GUI). They are used by different operating systems, eg Microsoft Windows.
The graphics can be in different resolutions and color depths. The most common are 16x16, 32x32 and 64x64 pixels. Color depths from 16 colors (4 bits) to true colors (24 bits) are possible.
In addition to the normal graphics programs that can handle ICO, there are also special software that is designed to edit and create icons.