Different forums have different rules and most require you to register as a member in order to view or post any entry. These are called threads. People can post replies to existing threads or start new threads.
One of my friends told me that online forums are a waste of time. I totally disagree with that. You can receive a lot of help from them. They are also really good networking tools.
Joining a forum depends on your particular area of interest. For example if you are a antique car enthusiast and looking for information on a particular make or even looking for parts, there might be some people on the forum which might point you in the right direction. People who are really into poker can discuss strategies, tournaments, etc. with like minded people in the Poker Forum on the site.