nostr exists out of smart clients and dumb relays. the messages of those postrs (nostr user is called postr or nostrich) get send to a relay and they send it back to everyone else, they relay stuff.

you have multiple clients for Nostr,

the pipular one being

amethyst

damus

plebstr

coracle

iris

snort

you do not need any sign up, the client will generate a keypair for you to sign your message. This is amazing as it doesmt require kyc.

Nostr as no algo rithm. you just get to see what reaches your client.

you can also connect your lightning wallet to nostr and send so called zaps, where you send real sats (smallest unit of Bitcoin) to people you like.