The Forums option in Moodle is a great way to carry on a conversation or discussion online.  Students could have to respond to a question, pose an idea, work collaboratively to answer a question or just even ask questions of each other and the teacher.  Forums in Moodle are controlled by the teacher and it can be very easy to delete inappropriate posts by students, a feature not found on other sites.  I have used Moodle Forums in the past to allow students to easily ask questions they have, which others can see since they may also have the same question.  This also is a great opportunity for those students that have trouble speaking out in class to be involved, they have more confidence when online as opposed to in the classroom.  Watch the video below for the basics of creating Forums in Moodle.