Minecraft wiki entry for Redstone_Torch, associated text: While active, a redstone torch:
powers adjacent redstone dust to power level 15, including beneath the redstone torch if it is attached to the side of a block
powers adjacent redstone comparators or redstone repeaters facing away from the redstone torch to power level 15
strongly powers a full solid opaque block above the redstone torch to power level 15 (but not next to or below it)
activates adjacent mechanism components, including above or below, such as pistons, redstone lamps, etc.
produces "reddust" particles