Kbin is a part of the Fediverse and is similar to Lemmy. I have a kbin.social account and am replying to you from kbin. (I subscribe to a lot of Lemmy communities via kbin).
Tildes is not a part of the Fediverse. It is a text-driven private forum basically created and run by one person. You need an invite from a Tildes account holder to join. It's its own little island. am on Tildes a lot and really like it.