Nostr: What is Nostr?

Short answer

Nostr is an open protocol for publishing signed messages through independently operated relays. People use clients to create and read those messages, and their identity is tied to a cryptographic keypair rather than one company's account database. It supports more than social posts, but each feature depends on the clients and relays that implement it. No single app or relay represents the entire network.

Sources

NIP-01: Basic protocol, Nostr Improvement Proposals index.