From The Art and Popular Culture Encyclopedia
In linguistics, the topic (or theme) of a sentence is often defined as what is being talked about, and the comment (rheme or focus) is what is being said about the topic. This division of the information structure of the sentence is generally agreed upon, but further than this the definition of what constitutes a "topic" is controversial and depends on which particular grammatical theory is being employed.
The difference between "topic" and grammatical subject is that topic is used to describe the information structure, or pragmatic structure of a sentence and how it coheres with other sentences, whereas the subject is a purely grammatical category. For example it is possible to have sentences where the subject is not the topic, for example often the case in passive sentences. In some languages word order and other syntactic phenomena is determined largely by the topic-comment structure, rather than by the grammatical structure of the sentence. These languages are sometimes referred to as topic-prominent languages. Chinese is often given as an example of this.
The distinction was probably first suggested by Henri Weil in 1844. Georg von der Gabelentz distinguished psychological subject (roughly topic) and psychological object (roughly focus). In the Prague school, the dichotomy, termed topic–focus articulation, has been studied mainly by Vilém Mathesius, Jan Firbas, František Daneš, Petr Sgall and Eva Hajičová. They have been concerned mainly by its relation to intonation and word-order. The work of Michael Halliday in the 1960s is likely responsible for bringing the ideas to functional grammar.
Note that in some categorizations, topic refers only to the contrastive theme and comment to the noncontrastive theme + rheme.