Relays are like postal services that make copies of your messages and distribute them when your followers ask for them.

They also give you copies of messages they got from people you follow.

In other words, they are public postal services.

Ironically, it's more similar to postal services than "email" (you don't need "user and password" to send/receive physical mail).

And clients are like typewriters. Apps like Damus, Amethyst and Nostrudel are "clients".

So, when you communicate through Nostr, you're communicating with the world, or at least the portion of it that you follow/follows you.