Basically you now have a globally unique identifier and a secret key that you keep private or only share with an application to log in.

Your messages are broadcast to one or more relays that are all run by different people and they largely don’t sync everything. So make sure you send to around 5 or more.

You can log into any Nostr supporting App with this identity. You data is stored on the app, but also relays (at least for a period of time). No client app or relay can censor you from the whole network.

And your data is in the network and always owned by you - you can download archives from a few services and upload to again if ever needed.