A discourse community is pretty much what the term suggests, namely a community (group of people) who are engaged in discourse (conversation). So, for example, an academic journal on a particular subject (Victorian Literature, let's say) would constitute a discourse community composed of all of the authors who contribute to the journal, as well as the journal's editors and its readers. One of the important things to remember about a discourse community is that such a community exists as a community by virtue of shared rules about who can say what how. So, a journal that only ever publishes, say, articles by people who have PhD's, ensures that only fairly elite voices get to speak in that journal's discourse community. Likewise, a journal might only publish pieces that are written in academic English, or that espouse mainstream theories, or that focus almost entirely on fringe theories. In each case, the rules act as limitations that serve to focus the exchange of information into a true discourse, a conversation rather than a series of voices talking across each other unintelligibly.
In your coursework, you will probably study the discourse community called the "public sphere," which is the biggest discourse community them all! Also, EssayForum can be considered a discourse community.
Great question.
