0xtr on Nostr: So nostr is a protocol and noStrudel, Damus, etc, are applications built on top of ...

So nostr is a protocol and noStrudel, Damus, etc, are applications built on top of this protocol. The protocol consists of something called NIPs (Nostr Implementation Possibilities) which are documents that define a certain behaviour for e.g. communities, "tweets", likes(reactions), etc. This makes it easy for multiple different clients to behave the same way and ensure interoperability. Communities is one of these NIPs (NIP-72) which defines how communities and related actions should behave and all clients that implements support for communities need to follow this document.

https://github.com/nostr-protocol/nips/blob/master/72.md

So to answer your question: yes, communities is different from nostrudel because nostrudel is "just a client" built on top of the nostr protocol while communities is _a part of_ the nostr protocol.

I hope that made it clear, if not please tell me what you didn't understand and I'll try to explain it better.