What are Reddit's user-created topic boards called?

Reddit’s user-created topic boards are called subreddits.

A subreddit is a community organized around a subject, interest, activity, or theme. Its name normally begins with “r/,” as in r/science, r/movies, or r/AskReddit. Users submit links, images, questions, and text posts, while voting and discussion help determine what appears prominently in the community.

Each subreddit has its own rules and volunteer moderators. Those moderators can remove posts, manage discussions, and set participation requirements, while Reddit’s administrators oversee the platform as a whole. This layered structure is why Reddit is often described as both a social-news site and a collection of forums.

“Boards” is an understandable generic description, but it is not Reddit’s standard term. “Channels” is more closely associated with services such as Slack or Discord, and a “thread” usually means a discussion attached to a particular post rather than an entire Reddit community.