DMs Direct messages work like Signal. There's no hosting keyserver, and each message is individually sent to every other chat participant.
Communities Communities are hosted on a particular user's keyserver, who serves as that community's root admin. Communities contain a tree structure of channels and support more sophisticated functionality than DMs.
Channels Channels are chats that exist inside a community. It's sort of like how channels work in Discord or Slack, with one important exception: channels can be nested inside other.
Threads Threads in Comm are pretty similar to threads in Discord or Slack. Both DMs and channels support threads, and threads appear in our unified inbox below their parent chat. Threads inside a channel can be "promoted" to a full channel.
