The tools available to you will differ from Course to Course depending upon what has been enabled/disable by your lecturer.
Discussion boards can play a vital part of asynchronous communication between students, students and instructors and for collaborative communication between groups. How the discussion board works is dependant upon how it has been set up in any given course. You may find that discussion boards are different from module to module depending upon the intended use.
There are many facets to a discussion board that should be understood in order to get the most from the tool.