My understanding is that Nostr isn't any specific site or service, but rather a web framework, kind of like Lemmy.
However, unlike Lemmy, which is hosted by individual servers running their own instances of the Lemmy software that can communicate with each other but are still their own unique entities, Nostr seems to be taking a more blockchain-like approach to decentralization, from my understanding of their docs. I don't think it's actually using blockchain tech, though, but looks somewhat similar in principle. It looks like people can set up "relays" which seem to act as verifiable mirrors/backups of anything posted through Nostr's network, which makes me think it operates not so much like blockchain, but more like Tor/onion networks.
That said, I clearly don't know how it works. But their big selling point seems to be that it's apparently "resistant to censorship", and they focus a lot on network security, encryption, etc. So it sounds like it's largely targeted toward people who are afraid of being "cancelled" on other platforms.
It makes me a bit worrisome about the risks a relay operator would be taking on, though. The people who seem to flock to these sorts of "free speech" platforms are generally the type to post dangerous or illegal content in general. Does running a Nostr relay put you at risk of hosting extremist or illegal content? With normal platforms, a hosting admin can remove what they want, but is that still doable with this so-called "censorship resistant" platform? It sounds like a liability nightmare.