The GameDev.tv forum implement a two level hierarchy to separate topics based on their subject matter. The terms category and sub-category are often used interchangeably with forum and sub-forum.
In the following screenshot UNITY is a category;
Within the UNITY category, sub-categories are listed, e.g. Showcase, S01 Intro, S02 NumberWizard…
Topics can be posted to either of these hierarchies, however, in practice topics are typically posted to sub-categories. A topic posted within the main category often indicates that the user wasn’t sure where to post their topic.
See also;