Nostr has two parts: Clients and Relays.

A CLIENT is an INTERFACE (app or website) that is run on the nostr protocol.

It is where you see the notes that you and the people you follow post (in the same way that twitter is an interface where you post and read notes by others, except twitter is centralized & it censors posts.)

A RELAY is a SERVER and a DATABASE. Anyone can run a relay, which is what makes nostr decentralized.

It is where your notes are sent, stored and retrieved from by clients.

There are many relays and you can choose which ones to connect to. Some are free and some are paid.

When you post a message, it is broadcast to the relays you are connected to.

The clients query the relays they are connected to, and then they populate the messages being hosted by those relays.