: What is a forum? What is a category?

Your message board can contain multiple forums. Each forum can contain multiple threads (or "topics"). Each thread can contain multiple replies. Categories are used to simply organize your forums.

For example:
You can have a forum called "Apples" where users would chat about different kinds of apples, a forum called "Oranges" where they would chat about different kinds of oranges, and a forum called "Bananas" where they would chat about different kinds of bananas.

You could then create a Category called "Fruits" and put all those forums into that category. The category would just be used to organize those forums. Categories are very helpful when you have many forums. It allows the user to easily choose the forum that is appropriate for their discussion.

Message Board
    Category: Fruits
               Forum: Apples
                       Thread: What are your favorite types of apples?
                       Thread: Do you like apples? 
               Forum: Oranges
                       Thread: Orange talk
    Category: Cars
               Forum: Car Discussion
                       Thread: I love Honda
                       Thread: My car broke down!
               Forum: Car Picture Post
                       Thread: Check out my car!