They are both relative to the perspective of the speaker. "Come back" implies that someone is going to return someplace they have been before. If in a conversation, it is to the place where the person is currently located. Example: "When will you come back home?" (The person asking the question is at home). "Next week." Or, "I had a great time in San Francisco. I hope I will be able to come back next year." (The speaker is in San Francisco, but will have to leave. He wants to return later). But if the speaker is NOT in San Francisco anymore, he could say "I hope I will be able to go back next year." "Go back" implies that the speaker is currently not in the place being talked about. "In February, I plan to go back to China" means right now I am not in China, but I have been there before and I plan to return in February. So, my friends in China might ask, "When do you plan to come back?" I could respond, "I plan to go back in February." You could also drop either and say "return" (When do you plan to return? I plan to return in February). These might help: http://www.bbc.co.uk/worldservice/learningenglish/youmeus/learnit/learnitv135.shtml or http://www.englishforums.com/English/ToComeBackVsToGoBack/vzqwm/post.htm

come back -- the direction of movement is here go back -- the direction of movement is away

It's related with the place you are :-) Example: You live in Philippines and are visiting a friend in London. You: I go back home to Philippines tomorrow. Friend: I'll miss you. Please come back { to visit London} next year. Teacher to a student in the hall Teacher: John, go back to your class! Student walks away. Teacher then says: John, come back here! I'm not finished talking to you.