In case you really don't know how forums work, here you go:
Forums: the name for the forums as a whole.
Subforums: the forums with titles like "General Discussion" or "Debate and Discussion."
Categories: the subforums are organized into these, they're not too important, just a thing.
Threads: these are the topics inside each subforum- it's the list of things which comes up when you go into a subforum. Each thread is its own topic- you know what it's about because of the usually helpful subject/title! Threads are listed inside a subforum in descending order of most recent post first. That means that the most active threads stay on the first page and old ones eventually move down.
Posts or replies: the bits of discussion going on within each thread. Replies in forums show up in chronological order, most recent on the last page.
If you're in a subforum, you can choose threads to read or create a new thread for a new topic. Make sure you're in the right place – if you want to make a thread about a movie, put it in the Media Megaforum.
Here's what a forum thread looks like without any replies:
You can see there's a lot of information here. Along the top, under "General Discussion" you can see a "breadcrumb trail" of where you are in the forums. If you want to go back up a level you can just click on it there. You can also search the topic (if there's a lot of replies/posts), or create your own reply. Note that you can do that at the top and bottom of each page!
Since I created this thread, I can edit its title/subject line. Since I'm an admin, I can delete it! Deleting the entire thread is not available to most users.
If you created a reply, you'll see an edit and delete button for your posts. That lets you fix mistakes, delete a doublepost, etc.
Someone who wants to quote what I said can do so with the "quote" button (quotations only go one level "deep", so quoting someone who quoted someone else won't work). If you want to link to that specific post out of the whole thread, use the "link" button.
There's also a poster
information area on the left hand side. It shows the username and avatar of
whoever made that reply, as well as some basic information about them and a
link to send them a PQ. Their signature is inserted below that horizontal line.
(You can see why we like to keep signature images small- seeing big ones over
and over again gets pretty annoying.)