There are several different types of forum to choose from:
A single simple discussion - is just a single topic, all on one page. Useful for short, focussed discussions.
Standard forum for general use - is an open forum where any one can start a new topic at any time. This is the best general-purpose forum.
Each person posts one discussion - Each person can post exactly one new discussion topic (everyone can reply to them though). This is useful when you want each student to start a discussion about, say, their reflections on the week's topic, and everyone else responds to these.
(More development to come in future versions of Moodle)