Urbit is a new decentralized global computing infrastructure. Urbit is three layers: a continent (Azimuth), a computer (Arvo), and a civilization (Casbah).
To oversimplify: Casbah is a decentralized social network between Arvo servers with Azimuth addresses.
What is the actual status of these things?
Azimuth, a civil address space, is done. It's live now on the Ethereum mainnet. Arvo, a typed functional OS, remains experimental; our project for 2019 is self-hosting development. Casbah is just our vision of how human beings will use Arvo; that vision won't be tested until Arvo is in production.
Urbit is a well-formed stack. No layer depends on a higher layer. All three parts of Urbit work well as part of one design; all three are independent things. Everyone should use them all; any can succeed, at least in some sense, without the others.
Azimuth is just a blockchain PKI; its keys can secure any protocol between any endpoints on any network. Arvo is just an open-source interpreter stack; anyone can use Arvo, Hoon, Nock or Vere for anything. Casbah is just an idea, or perhaps an ideal; the same ideal can inspire anyone.
And of course, each layer can be easily understood on its own. In separate essays, we describe the structure of Azimuth; the architecture and status of Arvo; and the vision of Casbah.