Minecraft wiki entry for Redstone_components, associated text: Mechanism components are turned on by:
an adjacent active power component (Exceptions: a redstone torch does not activate a mechanism component it is attached to, and a piston is activated only by a power component directly in front of it if the component is connected to it.)
an adjacent powered opaque block (strongly-powered or weakly-powered)
a powered redstone repeater or redstone comparator facing the mechanism component
powered redstone dust configured to point toward the mechanism component (or on top of it, for opaque mechanism components); a mechanism component is not turned ON by adjacent powered redstone dust that is not configured to point toward it.