This is the most asked question about Korean ever. Just do some googling and you will find lots of info. You can search here on italki too. And make sure you listen to this lesson about 는/ 은 & 이/가 http://www.talktomeinkorean.com/lessons/l1l9/

It's a matter of Topic Particle and Identifier Particle....:) They both are put into the subject but they differ on the emphasis of the subject of the sentence. :) To find out more, go to this link: http://parksguide.blogspot.com/2006/11/party-with-particles.html