It is a client (app) built on the Nostr protocol which is free and open source like bitcoin. It is similar to twttr in that you follow/have followers and post and like others posts etc.

It is very different in that it is not a walled garden, has no ‘owners’ and is run on a distributed network of voluntary relays. It cannot be shut down :)

It has nothing to do with crypto, but many clients integrate bitcoin Lightning wallets so people can ‘zap’ (send sats to) each other.

The 2nd edition of my book has a section on Nostr, free download available at thesimplestbitcoinbook.net 🤙