m (I changed the exhaust damage reduction in the first section of the page to 40% to match the actual amount. It says 40% in other sections of the page, but it seems someone simply forgot about that intro section.)
Exhaust is a targeted Summoner spell that affects enemy champions. It reduces damage dealt by 40% and reduces attack speed and movement speed by 30%.
Exhaust is an extremely powerful tool for disabling champions who mainly autoattack, as well as any champion that can deal a high amount of damage. When your team is chasing an enemy and he is getting away, using Exhaust on him will help your team gain the upper hand. Using it against enemies when escaping can help you as their movement speed will be decreased.