If you have a Nextcloud server and I have a Nextcloud server, how do we get them to communicate?
Federation is just a term for multiple instances of a thing connecting together through a common protocol despite them actually being separate.
Nextcloud federation currently allows two or more Nextcloud instances to share files and folders remotely. It is mostly a fancy word for sharing between Nextclouds.
Circles app is actively developing group support for federated circles, but it is still very much in development.
It works with some caveats…
Federated shares remain solely on their host server, but they will sync to your client devices.
Basically all apps in Nextcloud apart from Files and Circles do not support federation as of 2022, but they certainly hope to one day.
…Fediverse is a completely different thing.
Fediverse - Activitypub based social apps such as Mastodon, Pleroma, etc. are popular. Nextcloud has developed the Social app which is a chat application that can communicate with other activitypub chat applications. The Social app currently has nothing to do with the Nextcloud federation described above and has received no major releases since 2020.
Another useful way to allow your Nextcloud to communicate with other services is through webhooks. This is your MediaWiki site can auto-notify your Slack / Discord / Slack channel every time someone creates a new wiki page, despite them not being users on your wiki.
Nextcloud Flow webhooks hasn’t received any updates since version 20 and is not yet in the app store. It likely needs a new maintainer.
If it receives some love in the future webhooks could become an integral part of letting your Nextcloud share with not just other Nextcloud services, but whatever other service you are running, such as IFTTT, Zapier, etc.
RSS Feeds are another way to distribute and communicate with external services from Nextcloud. Currently you can optionally enable RSS Feeds from the Activity app and Bookmarks app, then plug them into RSS Bots on other services, including someone else’s Nextcloud instance running the News app.
The Dashboard app could become a lot more decentralized if it allowed the ability to generate RSS feeds, which you could then share with anyone. This could be especially useful if it related to particular projects being viewed and shared from the Dashboard.