Discourse is a fantastic platform to hold conversations: you can think of it as a mix of old-style forums, mailing lists, chat and messenger applications. The old threads are called topics and you can think of the old sub-forums as categories. There are a lot less categories (sub-forums) to choose from: #customer-discussion to hold most conversations, #suggestions to suggest new features or highlight pain points of DreamHost products, #site-feedback to discuss the forum itself.
The best way to understand all this is to send a direct message to @discobot user: it's a bot programmed with a tutorial. You'll learn how to do basic things in Discourse, like optimizing your reading, how to format your comments, get notifications and more.
Why did we reorganize categories
The home page of the old forums had a long list of sub-forums, and didn't really show much content at first. Reading was behind many clicks and also posting new questions was confusing. This new forum is the opposite: the home page is a long list of recent posts, sorted in reverse chronological order. To post new topics, one has few very clear options to pick the category.
The old posts have been migrated, and the sub-forums have been transformed into tags. For example, all posts in the old WordPress are now tagged #wordpress and filed in the main category #customer-discussion. Have a look and try to guess what other tags exist.
The other categories are #suggestions and #site-feedback. We use the former to collect feature requests for DreamHost products and the latter for comments, questions and tutorials on how to use this forum.
More categories will be added in the future for very specific content. Stay tuned, things will keep on changing for the best.