My favorite analogy is email. Even though there are lots of differences and nuances, i think it makes it easy to understand. A communication protocol on the internet, that can be built on top of. Think of your clients on nostr like yahoo, gmail, aol, etc. you can access the email protocol with any of those “clients” and send messages across clients. But on nostr, instead of sending a message to a predetermined recipient, it’s sent to a global timeline based on your relays. Another difference is that your email service stores your emails/data, but here it’s the relays that act in this manner, and the relays are independent from the clients.

This is extremely oversimplified, but I think it’s a good big picture view.