One of the most lethal yet subtle tools at your disposal on League of Legends is wards. Say you buy 3 for 225g, and place them correctly. You spot an enemy coming in to gank, and avoid dying. The warding has already more than paid for itself. Even though they have a fairly short duration, they can give you easy ganks. It's not uncommon to see teams grab aces because of a single ward at the Baron. This is a short guide on how to use your little buddies efficiently.
Here is a comprehensive image with warding locations I found in the backwater pages of google. You may have to save the image to see it properly. :
