Google Communities allows you to post any discussion to either the discussion stream or any of the other Section level streams.
Traditional forum works like a file system. Posts are placed in different categories. You can access them only if you are in that category.
Google community works like a tag system. You can access all posts at one time and you can also choose different tags to filter what post you want to see.
The category pandel is on the left. Select the category you want to see and click on it.
Now all the posts under this category are displaiyed on the right. You can click "All posts" to back.