Reddit users run the sections, and anyone can make their own. Each sub-Reddit is run by a group of users who either founded the sub-Reddit, or were later added by its founders.
Topics are called sub-Reddits, areas that are a part of Reddit, but are centered around a single genre or theme.
As you discover more niche communities, you can subscribe to those subreddits, see how many other redditors (Reddit users) are subscribed and how many are browsing the subreddit at the same moment .
If you subscribe to a subreddit the popular posts from those channels will be pushed directly to your homepage (The Front Page). You can always browse those individual subreddits directly, of course.