An article in the Windows Vista Team Blog shows the various types of UAC prompts and in which case (singed or unsigned executable etc.) which type is displayed. The screenshots are a bit tiny but you get the idea.
By default, other applications cannot send messages to the UAC dialog because it's displayed on the "secure desktop", which is basically a screenshot of your real desktop (see the clock in the system try doesn't update). This is to improve security. However in some cases it is necessary to "remote control" the UAC dialog, like a screen reader for visually impaired users. This can be done by manifesting the application, as described in the article UI Automation Security Overview on MSDN.