Introduction to Bulletin Board
The entire messenging system here which members can communicate by exchanging text messages is called a bulletin / message board. It is often interchangeably(wrongly) used with the term forum.
Within a board, there can different categories to segregate different types of discussion, these are called forums. Here within each forum, users can create a topic(also known as "thread") for discussion. Each reply to a topic is called a post.
So the hierarchy should look something like this:

Categories / Forums (eg. The Common Fishing Hole)
Threads / Topics (eg. Fishing Trip to Sedili)
Posts (eg. Re: Fishing Trip to Sedili)
Image hotlinking: To embed a picture in a post (often resided/stored in another website location)
Emoticons: They're smilies or small little faces/icons to add a little life to your posts. Don't overuse them.
Username: Your unique identification here on the bulletin board, you choose your username upon registering as a member.
Avatar: A small picture that appear just under your username. They don't mean much, more for decorative purpose and to fill up an empty space. You can choose an avatar in the galleries from your "profile".
Signature: A signature is something you added just below every post you make. You can place your name, some inspirational quotes, a link to your web or just leaving it blank. You can edit your signature in your "profile".
Private Messages (PM): A internal message system much like an email system where you can send to members of the bulletin board in private. Use it to discuss matters that only concern you and another party.
Moderator: They are members (usually volunteers) who help look after the forums. Ie. Making sure there are no inappropriate topics/posts.
Administrator: Takes care of the entire bulletin board, especially with its technical aspect, making sure that the board is up and running smoothly.