Minecraft wiki entry for Redstone_mechanics, associated text: an adjacent active power component, including above or below.
Exception: A redstone torch will not activate a mechanism component it is attached to
Exception: A piston is not activated by a power component directly in front of it.)
an adjacent powered opaque block (either strongly-powered or weakly-powered), including above or below.
a powered redstone comparator or redstone repeater facing the mechanism component.
powered redstone dust configured to point at the mechanism component (or on top of it, for mechanical components that can support redstone dust, but not beneath it), or adjacent "directionless" redstone dust; a mechanism component is not activated by adjacent powered redstone dust that is not configured to point at it