Forum is an online meeting place where several people interact with each other. This is different from a chat because on a forum although you dont see the updates in real time, they are stored on the site so that users can respond to each other's comments irrespective of whther they were online or not when the original message was posted..
Forums on this site are classified into several caterories. Each caterory is a place for a different kind of topic. Under each topic, there are several 'threads' that hold individual discussions.
Forums provide a way to discuss a lot of topics simultanously in an organised way. In contrast Blogs provide a focused way to discuss a single topic. Depending on your need, you might choose a Forum or a Blog to initiate a discussion.
All users can initiate a forum thread and can respond to existing threads. But only admins have the ability to created new categories or topics (sub-forums).