Nostr is a protocol, or a set of rules about how the apps built-on this rule should communicate. It's not an app by itself. But as a protocol, it lets other clients/apps build on it.
Apps (clients) can be developed based on this Nostr protocol. The beauty is, these clients can communicate between them. So, it's like the email. Email has protocols different protocol that makes it work. And different apps or clients, like Gmail, Yahoo Mail, etc. that can communicate among them.
Nostr is just like that. But unlike the email, Nostr protocol allows you to build any type of clients. So you can have alternative to Facebook, Twitter, Instagram, TikTok here on Nostr. Primal, Damus, Amethyst, etc. are Nostr clients that are more geared towards microblogging, primarily. More like alternative to X (formerly, Twitter). We already have an alternative to Instagram and it's called Olas. We have alternative to Medium, or for long-form content, and for that you can use Habla, Yakihonne, etc. We have clients/apps for podcasts, music, etc. We have an alternative to Reddit, hacker news, etc. And it's just the start.
Good thing is, you can use your same identity to sign in to these apps. And your followers and other social graph will transfer between the apps. You can use all the apps/clients at the same time. You can't use the same profile across Facebook, Twitter and others. But here on Nostr, you can.
About relays: relays are what stores and serves your data. When you're reading something, e.g. others posts, photos, etc. using a client/app, it's fetching them from the relay set you're using. And the same goes for when you're writing something or publishing. Relays are just servers. But instead of one centralized servers, you have many here. And you have the freedom to choose the relays here. So, in case one relay bans you, or doesn't want to host your content, you can always use another one. Or multiple others.
So, these are everything, I guess, you need to know for now. Don't worry about terminology. Just explore #Nostr and interact with people. Be yourself. You can use your real name here, or stay anonymous. You can have more than one profile, but that's not needed here, as you can use the same profile for signing in to all the clients built on the #Nostr protocol. Use hashtags frequently. Search for people, hashtags, And enjoy Nostr.