We'll start from the top. First you have Main topics. Under the main topics, you will see two: "video making" and "what is a forum?".
Under each topic, you will find threads. Threads are subjects about the topic, such as "special effects" under "video making".
After clicking on a thread, you will find posts. The posts are discussions that you can then enter, and write messages for. Once you post, you will find that other users will be able to write back in reply to you. It is a very fast effective way to discuss over the internet!
I hope this has helped!
