A forum is a place where you can discuss a topic with other people. On some sites people play word games on forums.
If that doesnt help much, heres a definition:
(this kinda confuses me, but maybe it'll help you)
An Internet forum, or message board, is an online discussion site. From a technological standpoint, forums or boards are web applications managing user-generated content.
People participating in an Internet forum may cultivate social bonds and interest groups for a topic made from the discussions.
