Minecraft wiki entry for Quasi-connectivity, associated text: Exceptions: a redstone torch does not activate a mechanism component it is attached to, and a piston is not activated by a power component directly in front of it
Examples: Redstone torches don't power blocks that aren't above them, but can activate mechanism components in any space next to them. Levers and buttons don't power blocks they aren't attached to, but can activate mechanism components in any space next to them.