In linguistics, the topic, or theme, of a sentence is what is being talked about, and the comment (rheme or focus) is what is being said about the topic. This division into old vs. new content is called information structure. It is generally agreed that clauses are divided into topic vs. comment, but in certain cases, the boundary between them depends on the specific grammatical theory that is used to analyze the sentence.
The topic of a sentence is distinct from the grammatical subject. The topic is defined by pragmatic considerations, that is, the context that provides meaning. The grammatical subject is defined by syntax. In any given sentence the topic and grammatical subject may be the same, but they need not be. For example, in the sentence "As for the little girl, the dog bit her", the subject is "the dog", but the topic is "the little girl".
