From what I've gathered and seen in the past couple months or so, Nostr is like Mastodon in that it is decentralized, and a user-hosted protocol, except unlike Mastodon you aren't running your own server, you simply have ownership of your account no matter where you go with it. Users can transfer value between each other by sending small amounts of BTC directly via a zap button on posts, and content moderation is entirely up to you (this is something I'm still figuring out, such as how to create a policy to block out potentially illegal material)