Minecraft wiki entry for Daylight_Detector, associated text: Behavior
An active daylight detector:
powers adjacent redstone dust, including below it, and redstone comparators facing away from the daylight detector, to a power level that depends on the time of day, the weather, and the internal sky light level (see tables below)
powers adjacent redstone repeaters facing away from the daylight detector to power level 15
activates adjacent redstone mechanisms, including above and below, such as pistons, redstone lamps, etc.