AN OVERVIEW:
Online discussions serve the same purpose as those that occur in-class, or anywhere else for that matter. Ideas are brought up and kicked around, students share and examine individual insights and perspectives; theoretically, everyone gains a greater understanding of the subject-at-hand.
What's the difference? Physicality: Place and time. Online, a discussion does not require the physical presence of a group gathered together at a pre-arranged time. Online, real-time contraints become malleable. This is especially helpful to students involved in collaborative assignments who may find it difficult to coordinate schedules: they can exchange ideas without having to meet face-to-face.
With online discussion capabilities, teacher-to-student and student-to-student communications reach beyond the classroom expanding and reframing available teaching and learning moments. In addition, online discussions foster a sense of community—a sense of participation in a shared enterprise—when distance and other factors inhibit classmates from gathering on a regular basis.