Fortify was a 'non-targeted' summoner spell that buffs your turrets on a global scale. Fortify was removed from League of Legends on November 15th, 2011.
Fortify is mainly used defensively to prevent towers from taking damage and clearing creeps faster then normally. However it is also a very effective method to punish tower divers or quickly do damage to foes that got caught by pulls and taunts like Rocket Grab and Puncturing Taunt.
While fortified, a turret grants you assists and kills when it last-hits or assists in killing an enemy champion as if you were dealing the damage, and not the turret. This does not, however, apply to minion kills.