While "subject" is a well defined grammar term, "topic" is a loose concept to explain certain sentence structures. Examples: - 한국은 아시아에 있다: 한국 is the topic as well as subject. - 한국은 산이 많다: As for Korea, there are a lot of mountains. There are two ways to look at this sentence. 1) consider 한국 as the subject of the whole sentence and 산 as that of the sub-clause 산이 많다. 2) consider 한국 as the topic of the sentence and 산 as the subject. In this interpretation you have a topic distinct from the subject of the sentence. Another example: 1) 하늘에 별이 총총하다: The sky is filled with stars. 별: subject & topic. 2) 하늘에는 별이 총총하다: As for the sky, it is filled with stars. 하늘: topic; 별: subject. #1 is a plain sentence with nothing special. In #2) adding the topic particle 는 to the location particle 에 turns the adverbial phrase 하늘에는 into a topic. It makes it clear that the speaker wants to talk about the sky, not the stars. In most sentences however, the subject of the sentence is also the topic.

I don't worry about the official names of those particles. Just get a feel for when to use each. It's like trying to learn all the names for the different speech levels. It's easier just to learn when to use each, instead of learning the names.