Minecraft wiki entry for Lever, associated text: powers any adjacent redstone dust (including beneath the lever) to power level 15
powers any adjacent redstone comparator or redstone repeater facing away from the lever to power level 15
strongly powers its attachment block to power level 15 (only if the attachment block is a full solid opaque block)
activates any adjacent mechanism components, including above or below, such as pistons, redstone lamps, etc.
emits redstone particles to indicate that it is active (in Java Edition).