What is a terrorist? One mans terrorist may be another mans freedom fighter.
What is it that Osama wants? He wants freedom for his people... probably.
Why did the US become his target? Well... in an open letter, he stated that as a 'believer' it his his duty to fight in the cause of allah, and those that disbelieve (and are therefore the followers of satan) fight against him. Simple. He's Allah's Freedom fighter, and all us non-believers, are satanists worthy of his vengence.
Religion, politics, guns and bombs, a recipe for a great party.