From the Main Page:
What is PunBB?
In short, PunBB is a fast and lightweight discussion board. It is released under the GNU Public License. It's primary goal is to be the faster, smaller and less graphic alternative to otherwise excellent discussion boards like phpBB, Invision Power Board and vBulletin.
That should explain everything!
A fast, simple to use forum seems to be the goal, not bloat, graphics, and fluff.