Categories are the highest level container of board posts that we have in our system. It is responsible for grouping forums together in such a way that makes it easy for a user to locate what they are most interested in. At this level we will keep a count of how many threads and posts we have as well as who made the last post and when. The reason that we maintain counts at this level is to remove the need for us to perform the count each time we display the category data.
A forum is a container of threads. It is responsible for grouping together a more specific set of posts. This too keeps a count of thread and post counts so that we can see at this level how much data lies within, again without needing to sum the actual post and thread counts.