STL, abbreviation for stereolithrography, is an interchangeable file format that represents 3-dimensional surface geometry. The file format finds its usage in several fields such as rapid prototyping, 3D printing and computer-aided manufacturing. It represents a surface as a series of small triangles, known as facets, where each facet is described by a perpendicular direction and three points representing the vertices of the triangle.
Files with ICO extension are image file types used as icon for representation of an application on Microsoft Windows. These come in different size, colour support and resolution to suit the requirements of the display. Another similar image file format on Microsoft Windows is .CUR for cursor representation and defines a hotspot in the image header. On MacOS, ICNS file formats serve the same purpose as ICO files. Several online websites as well as applications provide the feature of creating such files and convert other image formats such as BMP, PNG, etc. to icon file format. The official IANA registered Internet media type for ICO files is image/vnd.microsoft.icon.
You can also convert STL into many other file formats. Please see the complete list below.