I like this analogy. Relays can be your own little community, as open as you'd like them to be. You can run a public one or a private one. You could run one for all of your girl friends if you wanted. For example, I run several.

wss://relay.nostrplebs.com for https://nostrplebs.com paying members. You have to sign up and use our service to use this relay.

wss://nostrelites.org is a web of trust relay. Meaning it's a relay for everyone that I follow and the people that they follow to use. It prevents spam. It's fee. You can add it. The name is a meme. We're not elitists.

And I have a private relay that I run at my house for all of my notes and events. It's kind of like a backup, but a private vault for all of my chats and DMs, etc. I can't be censored because I run my own relay. Your apps will connect to my relay to download my content.

Primal's relay is a bit different. They aggregate content from all other relays and filter out certain content. They also offer a paid premium relay.

We also have some great topic based relays or algo relays showing content from those topics or communities.

We have a lot of other large public relays and smaller community relays. Some apps help with discovery here and some do not.

https://nostr.watch is a nice tool to show most relays, but it doesn't tell you what they're used for. I guess thats kind of a know if you already know situation. We need better explores here 😩