George, who uses gmail ([email protected]) can send emails to onno, who uses outlook ([email protected]).

George sends the mail via gmail, which sends the mail to outlook, which sends it to onno.

This is only possible because gmail and outlook share the email protocol. They are “federated”.

If a lemmy.world user submits a comment to a lemmy.ml thread, the user sends it to the lemmy.world server, who will pass it along to lemmy.ml, which will show the comment to lemmy.ml users.

In theory, this should keep everything in sync. In reality, mismatches in protocol implementation and differences in blocked users/instances, create different views on the same thread when viewed from lemmy.world and lemmy.ml

For example: lemmygrad.ml could be blocked/defederated by lemmy.world, while lemmy.ml is still federated to both. When viewing a thread from lemmy.ml, you can see content from both lemmy.world and lemmygrad.ml, while the views from lemmygrad.ml and lemmy.world lack each others content.