According the official website, "Discourse is a simple, flat forum, where replies flow down the page in a line. The expandable replies are attached to the bottom and top of each post, so you can discover the fuller context of the conversation - without breaking your flow."
This rough tutorial is a step-by-step guide about the how to setup Discourse aimed to advanced Rails developers who have installed their own Rails apps before. if you are new to Rails, you are likely much better off with this Discourse Vagrant Developer Guide.
This post is heavily based on this document although we have added some comments in bold+italic (missing things in the original file or maybe just due to our machine conf.).