There is just "the protocol". In order to get a client or a relay they should implement (some of) the NIPs described on

GitHub

GitHub - nostr-protocol/nostr: a truly censorship-resistant alternative to Twitter that has a chance of working

a truly censorship-resistant alternative to Twitter that has a chance of working - nostr-protocol/nostr

Whatever else a client or relay adds is up to the developers of said client/relay. This might be nothing up to extreme cases of tracking. Therefore it would be wise to choose an open source client and relay so you can verifyyou are not running any kind of spyware.