Tool is a non-creatable object. References to non-creatable objects must be obtained through other objects.
A Tool acts as a button that allows further interaction with the application's display. The Zoom In command is a good example of a tool--you click or drag a rectangle over a map before the display is redrawn to show the map contents in more detail. Toosl can only be placed on toolbars.
To create a custom Tool object, implement both esriSystemUI.ICommand and esriSystemUI.ITool.