If you look at the following dictionary definitions you will find that the word "subject" has 16 noun definitions (i.e. meanings and uses), there are 5 adjective defintions and 5 verb defintions: http://dictionary.reference.com/browse/subject I think you are interested in the first defintion: 1. that which forms a basic matter of thought, discussion, investigation, etc.: a subject of conversation. The word "topic" has 4 noun defintions. I think you are interested in the first defintion: 1. a subject of conversation or discussion: to provide a topic for discussion. English speakers often use either "subject or topic" to describe something that they are talking about or thinking about. The two words are synonyms (words that have a similar meaning).

Topic is what it covers (categorically) and subject is what it contains (categorically). Their use is interchangeable in many situations. Let's change the topic and leave this subject :)