I found this really hard to understand at the beginning too, I still don't think I could give a clear explanation of the two, but I think this does quite well: http://ec.libsyn.com/p/2/9/d/29da01498def6976/ttmik-l1l9.pdf?d13a76d516d9dec20c3d276ce028ed5089ab1ce3dae902ea1d01cf813ed8cb5a46e9&c_id=1552135 if you want to see more from that website, their lessons are free at www.talktomeinkorean.com, it'd really helped me. Hope that helps!

Yes,they all mean "I" 나 - informal term 저 - more formal term 는 is a topic particle 내가 = 나 + 가 가 is an object particle