“Puerto Rico is different because it is actually part of the US, and it has a unique status because it’s not a state and it’s not a foreign country. It’s what is known as a commonwealth or a territory; and the way it’s set up—and the way it has existed for over 100 years—is that Puerto Rican residents are all US citizens, but they do not have to pay US federal taxes on their Puerto Rican-sourced income. They pay taxes on that income to the Puerto Rican government. And this arrangement allows for the Puerto Rican government to offer incentives to mainland Americans who move to Puerto Rico that pretty much no other jurisdiction can offer.”
http://www.internationalman.com/articles/the-biggest-opportunity-for-americans-to-reduce-their-taxes