First If you create a category you have to create a forum for it too.
It will ask if you want to choose yes and create any name forum.
You also have to link it to blank, and can have it follow any main forum on the tree, but I dont think a sub forum would be a good idea, but it might work.
A category creates a title "bar", a forum does not have a bar.
So a category is the top dog, and forums then go into the categories, and forums can have sub forums and topics.
These can all be edited moved or deleted in the admin panel using the icons on the far right, mouse over them to see what they do.