adhoc - these can be created on the fly by users (like most chat apps support)
org - system channels; membership directly managed (e.g. you add / remove users from admin)
group - system channels; membership is automatically managed based on group membership (e.g. add a user to "brand" group and they will also be in a channel if the brand group is subscribed)
managed - system channels, support oriented. These allow users to create 1:1 chats that look like DM but are with some internal group of users
data - data channels; these are used to send data objects in real time