Overview
These key elements of Discourse’s structure confused me for a while; in order to make good use of Discourse, you really need to understand them clearly.
Categories
A category is a collection of topics. To denote a category, use a ‘#’ such as for this one: #help:howto. You can get to all of the categories visible to you in the ‘hamburger menu’ (
) or via the home button (
) in the top left.
Categories can be completely public, accessible just to logged in users, or only visible to a select few - all controlled by groups. They can have their own style / format, and quite radically different functions. For example, you can have a category function as a:
- a fully functional sub-forum
- a document repository
- a wikipedia style knowledge depository
- a Trello / kanban - type board
- a Calendar
- a place to discuss boring / private stuff for a select few
Some of the categories in the Forum are invisible to most people as they are private or targeted.
Sub-categories
These are full categories, nested under a ‘parent’ category. This is useful to keep things with a similar focus organised together. For example, #help also includes #help:howto and the (invisible to all except @staff) #help:meta.
Groups
A group is a collection of people. Simple as that. To mention a group, use the ‘@’ - just as you do for people. You can look at the groups via the ‘hamburger menu’ () and can narrow down to the groups you are a member of using one of several filters.
They can contain any number of people, from one up to hundreds of thousands (theoretically). They are used to control access to various parts of the forum, communicate internally within the group, or control behind the scenes stuff - including access and visibility of categories.
The Group Inbox
Each group has its own inbox. This is a private space for just those group members and essentially functions as a mini-category. The function is similar to Facebook Groups.
You visit a group inbox by clicking on a group that you are a member of, or via your personal inbox which is accessed by your avatar and then clicking the 
twice (or once and then the little arrow down the bottom.
This group inbox is great for group communications out of the box and can be upgraded to a category without too much trouble if they become a bit constraining.
How to make them
You need the help of @support - just tag or message us, or post a topic or reply in #help and we’ll do our very best to accommodate your needs. We are very open to however you want to do things.